diff --git a/examples/interpret.rs b/examples/interpret.rs
index fad4c51..dfc10ab 100644
--- a/examples/interpret.rs
+++ b/examples/interpret.rs
@@ -2,7 +2,7 @@ extern crate parity_wasm;
 
 use std::env::args;
 
-use parity_wasm::{interpreter, ModuleInstanceInterface};
+use parity_wasm::{interpreter, CustomModuleInstanceInterface};
 
 fn main() {
     let args: Vec<_> = args().collect();
diff --git a/examples/invoke.rs b/examples/invoke.rs
index 22a47e1..7171d6b 100644
--- a/examples/invoke.rs
+++ b/examples/invoke.rs
@@ -2,7 +2,7 @@ extern crate parity_wasm;
 
 use std::env::args;
 
-use parity_wasm::{interpreter, ModuleInstanceInterface, RuntimeValue};
+use parity_wasm::{interpreter, CustomModuleInstanceInterface, RuntimeValue};
 use parity_wasm::elements::{Internal, External, Type, FunctionType, ValueType};
 
 
diff --git a/spec/src/run.rs b/spec/src/run.rs
index f212bdb..fc62b51 100644
--- a/spec/src/run.rs
+++ b/spec/src/run.rs
@@ -13,7 +13,8 @@ use parity_wasm::interpreter::{
     RuntimeValue,
     ProgramInstance, ModuleInstance, 
     ItemIndex, ExportEntryType,
-    Error as InterpreterError,
+    Error as InternalInterpreterError,
+    InterpreterError
 };
 
 fn spec_test_module() -> elements::Module {
@@ -60,7 +61,12 @@ fn try_deserialize(base_dir: &str, module_path: &str) -> Result<elements::Module
 fn try_load(base_dir: &str, module_path: &str) -> Result<(), parity_wasm::interpreter::Error> {
     let module = try_deserialize(base_dir, module_path).map_err(|e| parity_wasm::interpreter::Error::Program(format!("{:?}", e)))?;
     let program = ProgramInstance::new().expect("Failed creating program");
-    program.add_module("try_load", module, None).map(|_| ())
+    program.add_module("try_load", module, None)
+        .map(|_| ())
+        .map_err(|e| match e {
+            InterpreterError::Internal(e) => e,
+            _ => unreachable!(),
+        })
 }
 
 fn runtime_value(test_val: &test::RuntimeValue) -> parity_wasm::RuntimeValue {
@@ -90,7 +96,7 @@ fn runtime_values(test_vals: &[test::RuntimeValue]) -> Vec<parity_wasm::RuntimeV
 }
 
 fn run_action(program: &ProgramInstance, action: &test::Action) 
-    -> Result<Option<parity_wasm::RuntimeValue>, InterpreterError> 
+    -> Result<Option<parity_wasm::RuntimeValue>, InternalInterpreterError> 
 {
     match *action {
         test::Action::Invoke { ref module, ref field, ref args } => {
@@ -98,6 +104,10 @@ fn run_action(program: &ProgramInstance, action: &test::Action)
             let module = module.trim_left_matches('$');
             let module = program.module(&module).expect(&format!("Expected program to have loaded module {}", module));
             module.execute_export(&jstring_to_rstring(field), runtime_values(args).into())
+                .map_err(|e| match e {
+                    InterpreterError::Internal(e) => e,
+                    _ => unreachable!(),
+                })
         },
         test::Action::Get { ref module, ref field, .. } => {
             let module = module.clone().unwrap_or("wasm_test".into());
@@ -108,7 +118,7 @@ fn run_action(program: &ProgramInstance, action: &test::Action)
             module.export_entry(field.as_ref(), &ExportEntryType::Any)
                 .and_then(|i| match i {
                     elements::Internal::Global(global_index) => Ok(ItemIndex::IndexSpace(global_index)),
-                    _ => Err(InterpreterError::Global(format!("Expected to have exported global with name {}", field))),
+                    _ => Err(InternalInterpreterError::Global(format!("Expected to have exported global with name {}", field))),
                 })
                 .and_then(|g| module.global(g, None, None).map(|g| Some(g.get())))
         }
diff --git a/src/interpreter/env.rs b/src/interpreter/env.rs
index 6748f2b..e89e68a 100644
--- a/src/interpreter/env.rs
+++ b/src/interpreter/env.rs
@@ -3,7 +3,7 @@ use std::collections::HashMap;
 use builder::module;
 use elements::{Module, ExportEntry, Internal, GlobalEntry, GlobalType,
 	ValueType, InitExpr, Opcode, Opcodes};
-use interpreter::Error;
+use interpreter::{Error, CustomUserError, InterpreterError};
 use interpreter::env_native::NATIVE_INDEX_FUNC_MIN;
 use interpreter::module::{ModuleInstanceInterface, ModuleInstance, ExecutionParams,
 	ItemIndex, CallerContext, ExportEntryType, InternalFunctionReference, InternalFunction, FunctionSignature};
@@ -78,12 +78,12 @@ pub struct EnvParams {
 	pub allow_memory_growth: bool,
 }
 
-pub struct EnvModuleInstance {
+pub struct EnvModuleInstance<E: CustomUserError> {
 	_params: EnvParams,
-	instance: ModuleInstance,
+	instance: ModuleInstance<E>,
 }
 
-impl EnvModuleInstance {
+impl<E> EnvModuleInstance<E> where E: CustomUserError {
 	pub fn new(params: EnvParams, module: Module) -> Result<Self, Error> {
 		let mut instance = ModuleInstance::new(Weak::default(), "env".into(), module)?;
 		instance.instantiate(None)?;
@@ -95,12 +95,12 @@ impl EnvModuleInstance {
 	}
 }
 
-impl ModuleInstanceInterface for EnvModuleInstance {
-	fn execute_index(&self, index: u32, params: ExecutionParams) -> Result<Option<RuntimeValue>, Error> {
+impl<E> ModuleInstanceInterface<E> for EnvModuleInstance<E> where E: CustomUserError {
+	fn execute_index(&self, index: u32, params: ExecutionParams<E>) -> Result<Option<RuntimeValue>, InterpreterError<E>> {
 		self.instance.execute_index(index, params)
 	}
 
-	fn execute_export(&self, name: &str, params: ExecutionParams) -> Result<Option<RuntimeValue>, Error> {
+	fn execute_export(&self, name: &str, params: ExecutionParams<E>) -> Result<Option<RuntimeValue>, InterpreterError<E>> {
 		self.instance.execute_export(name, params)
 	}
 
@@ -116,7 +116,7 @@ impl ModuleInstanceInterface for EnvModuleInstance {
 		self.instance.memory(index)
 	}
 
-	fn global<'a>(&self, index: ItemIndex, variable_type: Option<VariableType>, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>) -> Result<Arc<VariableInstance>, Error> {
+	fn global<'a>(&self, index: ItemIndex, variable_type: Option<VariableType>, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>) -> Result<Arc<VariableInstance>, Error> {
 		self.instance.global(index, variable_type, externals)
 	}
 
@@ -128,11 +128,11 @@ impl ModuleInstanceInterface for EnvModuleInstance {
 		self.instance.function_type_by_index(type_index)
 	}
 
-	fn function_reference<'a>(&self, index: ItemIndex, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>) -> Result<InternalFunctionReference<'a>, Error> {
+	fn function_reference<'a>(&self, index: ItemIndex, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>) -> Result<InternalFunctionReference<'a, E>, Error> {
 		self.instance.function_reference(index, externals)
 	}
 
-	fn function_reference_indirect<'a>(&self, table_idx: u32, type_idx: u32, func_idx: u32, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>) -> Result<InternalFunctionReference<'a>, Error> {
+	fn function_reference_indirect<'a>(&self, table_idx: u32, type_idx: u32, func_idx: u32, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>) -> Result<InternalFunctionReference<'a, E>, Error> {
 		self.instance.function_reference_indirect(table_idx, type_idx, func_idx, externals)
 	}
 
@@ -140,12 +140,13 @@ impl ModuleInstanceInterface for EnvModuleInstance {
 		Ok(None)
 	}
 
-	fn call_internal_function(&self, outer: CallerContext, index: u32) -> Result<Option<RuntimeValue>, Error> {
+	fn call_internal_function(&self, outer: CallerContext<E>, index: u32) -> Result<Option<RuntimeValue>, InterpreterError<E>> {
 		// to make interpreter independent of *SCRIPTEN runtime, just make abort/assert = interpreter Error
 		match index {
 			INDEX_FUNC_ABORT => self.global(ItemIndex::IndexSpace(INDEX_GLOBAL_ABORT), Some(VariableType::I32), None)
 				.and_then(|g| g.set(RuntimeValue::I32(1)))
-				.and_then(|_| Err(Error::Trap("abort".into()))),
+				.and_then(|_| Err(Error::Trap("abort".into())))
+				.map_err(Into::into),
 			INDEX_FUNC_ASSERT => outer.value_stack.pop_as::<i32>()
 				.and_then(|condition| if condition == 0 {
 					self.global(ItemIndex::IndexSpace(INDEX_GLOBAL_ABORT), Some(VariableType::I32), None)
@@ -153,18 +154,20 @@ impl ModuleInstanceInterface for EnvModuleInstance {
 						.and_then(|_| Err(Error::Trap("assertion failed".into())))
 				} else {
 					Ok(None)
-				}),
+				})
+				.map_err(Into::into),
 			INDEX_FUNC_ENLARGE_MEMORY => Ok(Some(RuntimeValue::I32(0))), // TODO: support memory enlarge
 			INDEX_FUNC_GET_TOTAL_MEMORY => self.global(ItemIndex::IndexSpace(INDEX_GLOBAL_TOTAL_MEMORY), Some(VariableType::I32), None)
 				.map(|g| g.get())
-				.map(Some),
-			INDEX_FUNC_MIN_NONUSED ... INDEX_FUNC_MAX => Err(Error::Trap("unimplemented".into())),
-			_ => Err(Error::Trap(format!("trying to call function with index {} in env module", index))),
+				.map(Some)
+				.map_err(Into::into),
+			INDEX_FUNC_MIN_NONUSED ... INDEX_FUNC_MAX => Err(Error::Trap("unimplemented".into()).into()),
+			_ => Err(Error::Trap(format!("trying to call function with index {} in env module", index)).into()),
 		}
 	}
 }
 
-pub fn env_module(params: EnvParams) -> Result<EnvModuleInstance, Error> {
+pub fn env_module<E: CustomUserError>(params: EnvParams) -> Result<EnvModuleInstance<E>, Error> {
 	debug_assert!(params.total_stack < params.total_memory);
 	debug_assert!((params.total_stack % LINEAR_MEMORY_PAGE_SIZE) == 0);
 	debug_assert!((params.total_memory % LINEAR_MEMORY_PAGE_SIZE) == 0);
diff --git a/src/interpreter/env_native.rs b/src/interpreter/env_native.rs
index 921067f..22a7955 100644
--- a/src/interpreter/env_native.rs
+++ b/src/interpreter/env_native.rs
@@ -3,7 +3,7 @@ use std::collections::HashMap;
 use std::borrow::Cow;
 use parking_lot::RwLock;
 use elements::{Internal, ValueType};
-use interpreter::Error;
+use interpreter::{Error, CustomUserError, InterpreterError};
 use interpreter::module::{ModuleInstanceInterface, ExecutionParams, ItemIndex,
 	CallerContext, ExportEntryType, InternalFunctionReference, InternalFunction, FunctionSignature};
 use interpreter::memory::MemoryInstance;
@@ -17,9 +17,9 @@ pub const NATIVE_INDEX_FUNC_MIN: u32 = 10001;
 pub const NATIVE_INDEX_GLOBAL_MIN: u32 = 20001;
 
 /// User functions executor.
-pub trait UserFunctionExecutor {
+pub trait UserFunctionExecutor<E: CustomUserError> {
 	/// Execute function with given name.
-	fn execute(&mut self, name: &str, context: CallerContext) -> Result<Option<RuntimeValue>, Error>;
+	fn execute(&mut self, name: &str, context: CallerContext<E>) -> Result<Option<RuntimeValue>, InterpreterError<E>>;
 }
 
 /// User function descriptor
@@ -68,21 +68,21 @@ impl UserFunctionDescriptor {
 }
 
 /// Set of user-defined module elements.
-pub struct UserDefinedElements<'a> {
+pub struct UserDefinedElements<'a, E: 'a + CustomUserError> {
 	/// User globals list.
 	pub globals: HashMap<String, Arc<VariableInstance>>,
 	/// User functions list.
 	pub functions: Cow<'static, [UserFunctionDescriptor]>,
 	/// Functions executor.
-	pub executor: Option<&'a mut UserFunctionExecutor>,
+	pub executor: Option<&'a mut UserFunctionExecutor<E>>,
 }
 
 /// Native module instance.
-pub struct NativeModuleInstance<'a> {
+pub struct NativeModuleInstance<'a, E: 'a + CustomUserError> {
 	/// Underllying module reference.
-	env: Arc<ModuleInstanceInterface>,
+	env: Arc<ModuleInstanceInterface<E>>,
 	/// User function executor.
-	executor: RwLock<Option<&'a mut UserFunctionExecutor>>,
+	executor: RwLock<Option<&'a mut UserFunctionExecutor<E>>>,
 	/// By-name functions index.
 	functions_by_name: HashMap<String, u32>,
 	/// User functions list.
@@ -93,9 +93,9 @@ pub struct NativeModuleInstance<'a> {
 	globals: Vec<Arc<VariableInstance>>,
 }
 
-impl<'a> NativeModuleInstance<'a> {
+impl<'a, E> NativeModuleInstance<'a, E> where E: CustomUserError {
 	/// Create new native module
-	pub fn new(env: Arc<ModuleInstanceInterface>, elements: UserDefinedElements<'a>) -> Result<Self, Error> {
+	pub fn new(env: Arc<ModuleInstanceInterface<E>>, elements: UserDefinedElements<'a, E>) -> Result<Self, Error> {
 		if !elements.functions.is_empty() && elements.executor.is_none() {
 			return Err(Error::Function("trying to construct native env module with functions, but without executor".into()));
 		}
@@ -111,12 +111,12 @@ impl<'a> NativeModuleInstance<'a> {
 	}
 }
 
-impl<'a> ModuleInstanceInterface for NativeModuleInstance<'a> {
-	fn execute_index(&self, index: u32, params: ExecutionParams) -> Result<Option<RuntimeValue>, Error> {
+impl<'a, E> ModuleInstanceInterface<E> for NativeModuleInstance<'a, E> where E: CustomUserError {
+	fn execute_index(&self, index: u32, params: ExecutionParams<E>) -> Result<Option<RuntimeValue>, InterpreterError<E>> {
 		self.env.execute_index(index, params)
 	}
 
-	fn execute_export(&self, name: &str, params: ExecutionParams) -> Result<Option<RuntimeValue>, Error> {
+	fn execute_export(&self, name: &str, params: ExecutionParams<E>) -> Result<Option<RuntimeValue>, InterpreterError<E>> {
 		self.env.execute_export(name, params)
 	}
 
@@ -155,7 +155,7 @@ impl<'a> ModuleInstanceInterface for NativeModuleInstance<'a> {
 		self.env.memory(index)
 	}
 
-	fn global<'b>(&self, global_index: ItemIndex, variable_type: Option<VariableType>, externals: Option<&'b HashMap<String, Arc<ModuleInstanceInterface + 'b>>>) -> Result<Arc<VariableInstance>, Error> {
+	fn global<'b>(&self, global_index: ItemIndex, variable_type: Option<VariableType>, externals: Option<&'b HashMap<String, Arc<ModuleInstanceInterface<E> + 'b>>>) -> Result<Arc<VariableInstance>, Error> {
 		let index = match global_index {
 			ItemIndex::IndexSpace(index) | ItemIndex::Internal(index) => index,
 			ItemIndex::External(_) => unreachable!("trying to get global, exported by native env module"),
@@ -190,11 +190,11 @@ impl<'a> ModuleInstanceInterface for NativeModuleInstance<'a> {
 		self.function_type(ItemIndex::Internal(type_index))
 	}
 
-	fn function_reference<'b>(&self, index: ItemIndex, externals: Option<&'b HashMap<String, Arc<ModuleInstanceInterface + 'b>>>) -> Result<InternalFunctionReference<'b>, Error> {
+	fn function_reference<'b>(&self, index: ItemIndex, externals: Option<&'b HashMap<String, Arc<ModuleInstanceInterface<E> + 'b>>>) -> Result<InternalFunctionReference<'b, E>, Error> {
 		self.env.function_reference(index, externals)
 	}
 
-	fn function_reference_indirect<'b>(&self, table_idx: u32, type_idx: u32, func_idx: u32, externals: Option<&'b HashMap<String, Arc<ModuleInstanceInterface + 'b>>>) -> Result<InternalFunctionReference<'b>, Error> {
+	fn function_reference_indirect<'b>(&self, table_idx: u32, type_idx: u32, func_idx: u32, externals: Option<&'b HashMap<String, Arc<ModuleInstanceInterface<E> + 'b>>>) -> Result<InternalFunctionReference<'b, E>, Error> {
 		self.env.function_reference_indirect(table_idx, type_idx, func_idx, externals)
 	}
 
@@ -202,14 +202,14 @@ impl<'a> ModuleInstanceInterface for NativeModuleInstance<'a> {
 		Ok(None)
 	}
 
-	fn call_internal_function(&self, outer: CallerContext, index: u32) -> Result<Option<RuntimeValue>, Error> {
+	fn call_internal_function(&self, outer: CallerContext<E>, index: u32) -> Result<Option<RuntimeValue>, InterpreterError<E>> {
 		if index < NATIVE_INDEX_FUNC_MIN || index >= NATIVE_INDEX_GLOBAL_MIN {
 			return self.env.call_internal_function(outer, index);
 		}
 
 		self.functions
 			.get((index - NATIVE_INDEX_FUNC_MIN) as usize)
-			.ok_or(Error::Native(format!("trying to call native function with index {}", index)))
+			.ok_or(Error::Native(format!("trying to call native function with index {}", index)).into())
 			.and_then(|f| self.executor.write()
 				.as_mut()
 				.expect("function existss; if function exists, executor must also exists [checked in constructor]; qed")
@@ -218,7 +218,7 @@ impl<'a> ModuleInstanceInterface for NativeModuleInstance<'a> {
 }
 
 /// Create wrapper for env module with given native user functions.
-pub fn env_native_module<'a>(env: Arc<ModuleInstanceInterface>, user_elements: UserDefinedElements<'a>) -> Result<NativeModuleInstance, Error> {
+pub fn env_native_module<'a, E: CustomUserError>(env: Arc<ModuleInstanceInterface<E>>, user_elements: UserDefinedElements<'a, E>) -> Result<NativeModuleInstance<E>, Error> {
 	NativeModuleInstance::new(env, user_elements)
 }
 
diff --git a/src/interpreter/imports.rs b/src/interpreter/imports.rs
index 7243e27..e785cc3 100644
--- a/src/interpreter/imports.rs
+++ b/src/interpreter/imports.rs
@@ -1,7 +1,7 @@
 use std::sync::{Arc, Weak};
 use std::collections::HashMap;
 use elements::{ImportSection, ImportEntry, External, Internal};
-use interpreter::Error;
+use interpreter::{Error, CustomUserError};
 use interpreter::memory::MemoryInstance;
 use interpreter::module::{ModuleInstanceInterface, ItemIndex, ExportEntryType, FunctionSignature};
 use interpreter::program::ProgramInstanceEssence;
@@ -9,9 +9,9 @@ use interpreter::table::TableInstance;
 use interpreter::variable::{VariableInstance, VariableType};
 
 /// Module imports.
-pub struct ModuleImports {
+pub struct ModuleImports<E: CustomUserError> {
 	/// Program instance.
-	program: Weak<ProgramInstanceEssence>,
+	program: Weak<ProgramInstanceEssence<E>>,
 	/// External functions.
 	functions: Vec<usize>,
 	/// External tables.
@@ -22,9 +22,9 @@ pub struct ModuleImports {
 	globals: Vec<usize>,
 }
 
-impl ModuleImports {
+impl<E> ModuleImports<E> where E: CustomUserError {
 	/// Create new imports for given import section.
-	pub fn new(program: Weak<ProgramInstanceEssence>, import_section: Option<&ImportSection>) -> Self {
+	pub fn new(program: Weak<ProgramInstanceEssence<E>>, import_section: Option<&ImportSection>) -> Self {
 		let mut functions = Vec::new();
 		let mut tables = Vec::new();
 		let mut memory = Vec::new();
@@ -104,7 +104,7 @@ impl ModuleImports {
 	}
 
 	/// Get module reference.
-	pub fn module<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>, name: &str) -> Result<Arc<ModuleInstanceInterface + 'a>, Error> {
+	pub fn module<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>, name: &str) -> Result<Arc<ModuleInstanceInterface<E> + 'a>, Error> {
 		if let Some(externals) = externals {
 			if let Some(module) = externals.get(name).cloned() {
 				return Ok(module);
@@ -118,7 +118,7 @@ impl ModuleImports {
 	}
 
 	/// Get function index.
-	pub fn function<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>, import: &ImportEntry, required_type: Option<FunctionSignature>) -> Result<u32, Error> {
+	pub fn function<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>, import: &ImportEntry, required_type: Option<FunctionSignature>) -> Result<u32, Error> {
 		let (_, export) = self.external_export(externals, import, &required_type.map(|ft| ExportEntryType::Function(ft)).unwrap_or(ExportEntryType::Any))?;
 		if let Internal::Function(external_index) = export {
 			return Ok(external_index);
@@ -128,7 +128,7 @@ impl ModuleImports {
 	}
 
 	/// Get table reference.
-	pub fn table<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>, import: &ImportEntry) -> Result<Arc<TableInstance>, Error> {
+	pub fn table<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>, import: &ImportEntry) -> Result<Arc<TableInstance>, Error> {
 		let (module, export) = self.external_export(externals, import, &ExportEntryType::Any)?;
 		if let Internal::Table(external_index) = export {
 			return module.table(ItemIndex::Internal(external_index));
@@ -138,7 +138,7 @@ impl ModuleImports {
 	}
 
 	/// Get memory reference.
-	pub fn memory<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>, import: &ImportEntry) -> Result<Arc<MemoryInstance>, Error> {
+	pub fn memory<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>, import: &ImportEntry) -> Result<Arc<MemoryInstance>, Error> {
 		let (module, export) = self.external_export(externals, import, &ExportEntryType::Any)?;
 		if let Internal::Memory(external_index) = export {
 			return module.memory(ItemIndex::Internal(external_index));
@@ -148,7 +148,7 @@ impl ModuleImports {
 	}
 
 	/// Get global reference.
-	pub fn global<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>, import: &ImportEntry, required_type: Option<VariableType>) -> Result<Arc<VariableInstance>, Error> {
+	pub fn global<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>, import: &ImportEntry, required_type: Option<VariableType>) -> Result<Arc<VariableInstance>, Error> {
 		let (module, export) = self.external_export(externals, import, &required_type.clone().map(|rt| ExportEntryType::Global(rt)).unwrap_or(ExportEntryType::Any))?;
 		if let Internal::Global(external_index) = export {
 			return module.global(ItemIndex::Internal(external_index), required_type, externals);
@@ -157,7 +157,7 @@ impl ModuleImports {
 		Err(Error::Program(format!("wrong import {} from module {} (expecting global)", import.field(), import.module())))
 	}
 
-	fn external_export<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>, import: &ImportEntry, required_type: &ExportEntryType) -> Result<(Arc<ModuleInstanceInterface + 'a>, Internal), Error> {
+	fn external_export<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>, import: &ImportEntry, required_type: &ExportEntryType) -> Result<(Arc<ModuleInstanceInterface<E> + 'a>, Internal), Error> {
 		self.module(externals, import.module())
 			.and_then(|m|
 				m.export_entry(import.field(), required_type)
diff --git a/src/interpreter/mod.rs b/src/interpreter/mod.rs
index 3ebeccd..c369434 100644
--- a/src/interpreter/mod.rs
+++ b/src/interpreter/mod.rs
@@ -2,6 +2,19 @@
 
 /// Interpreter error.
 #[derive(Debug, Clone, PartialEq)]
+pub enum InterpreterError<E: CustomUserError> {
+	/// Internal error.
+	Internal(Error),
+	/// Custom user error.
+	User(E),
+}
+
+/// Custom user error.
+pub trait CustomUserError: 'static + ::std::fmt::Display + ::std::fmt::Debug + Clone + PartialEq {
+}
+
+/// Internal interpreter error.
+#[derive(Debug, Clone, PartialEq)]
 pub enum Error {
 	/// Program-level error.
 	Program(String),
@@ -35,6 +48,12 @@ pub enum Error {
 	Trap(String),
 }
 
+impl<E> From<Error> for InterpreterError<E> where E: CustomUserError {
+	fn from(other: Error) -> Self {
+		InterpreterError::Internal(other)
+	}
+}
+
 impl Into<String> for Error {
 	fn into(self) -> String {
 		match self {
@@ -57,6 +76,16 @@ impl Into<String> for Error {
 	}
 }
 
+/// Dummy user error.
+#[derive(Debug, Clone, PartialEq)]
+pub struct DummyCustomUserError;
+
+impl CustomUserError for DummyCustomUserError {}
+
+impl ::std::fmt::Display for DummyCustomUserError {
+	fn fmt(&self, _f: &mut ::std::fmt::Formatter) -> Result<(), ::std::fmt::Error> { Ok(()) }
+}
+
 mod env;
 mod env_native;
 mod imports;
@@ -74,10 +103,24 @@ mod variable;
 mod tests;
 
 pub use self::memory::MemoryInstance;
-pub use self::module::{ModuleInstance, ModuleInstanceInterface, ItemIndex, ExportEntryType, CallerContext, ExecutionParams, FunctionSignature};
+pub use self::module::{ModuleInstance as CustomModuleInstance,
+	ModuleInstanceInterface as CustomModuleInstanceInterface,
+	ItemIndex, ExportEntryType, CallerContext, ExecutionParams, FunctionSignature};
 pub use self::table::TableInstance;
-pub use self::program::ProgramInstance;
+pub use self::program::ProgramInstance as CustomProgramInstance;
 pub use self::value::RuntimeValue;
 pub use self::variable::{VariableInstance, VariableType, ExternalVariableValue};
 pub use self::env_native::{env_native_module, UserDefinedElements, UserFunctionExecutor, UserFunctionDescriptor};
-pub use self::env::EnvParams;
\ No newline at end of file
+pub use self::env::EnvParams;
+
+/// Default type of ProgramInstance if you do not need any custom user errors.
+/// To work with custom user errors or interpreter internals, use CustomProgramInstance.
+pub type ProgramInstance = self::program::ProgramInstance<DummyCustomUserError>;
+
+/// Default type of ModuleInstance if you do not need any custom user errors.
+/// To work with custom user errors or interpreter internals, use CustomModuleInstance.
+pub type ModuleInstance = self::module::ModuleInstance<DummyCustomUserError>;
+
+/// Default type of ModuleInstanceInterface if you do not need any custom user errors.
+/// To work with custom user errors or interpreter internals, use CustomModuleInstanceInterface.
+pub type ModuleInstanceInterface = self::module::ModuleInstanceInterface<DummyCustomUserError>;
diff --git a/src/interpreter/module.rs b/src/interpreter/module.rs
index 9327154..35fb8a2 100644
--- a/src/interpreter/module.rs
+++ b/src/interpreter/module.rs
@@ -3,7 +3,7 @@ use std::iter::repeat;
 use std::sync::{Arc, Weak};
 use std::fmt;
 use elements::{Module, InitExpr, Opcode, Type, FunctionType, Internal, External, BlockType, ResizableLimits, Local, ValueType};
-use interpreter::Error;
+use interpreter::{Error, CustomUserError, InterpreterError};
 use interpreter::env_native::UserFunctionDescriptor;
 use interpreter::imports::ModuleImports;
 use interpreter::memory::MemoryInstance;
@@ -21,12 +21,12 @@ const DEFAULT_VALUE_STACK_LIMIT: usize = 16384;
 const DEFAULT_FRAME_STACK_LIMIT: usize = 1024;
 
 /// Execution context.
-#[derive(Default, Clone)]
-pub struct ExecutionParams<'a> {
+#[derive(Clone)]
+pub struct ExecutionParams<'a, E: CustomUserError> {
 	/// Arguments.
 	pub args: Vec<RuntimeValue>,
 	/// Execution-local external modules.
-	pub externals: HashMap<String, Arc<ModuleInstanceInterface + 'a>>,
+	pub externals: HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>,
 }
 
 /// Export type.
@@ -50,11 +50,11 @@ pub enum FunctionSignature<'a> {
 }
 
 /// Module instance API.
-pub trait ModuleInstanceInterface {
+pub trait ModuleInstanceInterface<E: CustomUserError> {
 	/// Execute function with the given index.
-	fn execute_index(&self, index: u32, params: ExecutionParams) -> Result<Option<RuntimeValue>, Error>;
+	fn execute_index(&self, index: u32, params: ExecutionParams<E>) -> Result<Option<RuntimeValue>, InterpreterError<E>>;
 	/// Execute function with the given export name.
-	fn execute_export(&self, name: &str, params: ExecutionParams) -> Result<Option<RuntimeValue>, Error>;
+	fn execute_export(&self, name: &str, params: ExecutionParams<E>) -> Result<Option<RuntimeValue>, InterpreterError<E>>;
 	/// Get export entry.
 	fn export_entry<'a>(&self, name: &str, required_type: &ExportEntryType) -> Result<Internal, Error>;
 	/// Get table reference.
@@ -62,19 +62,19 @@ pub trait ModuleInstanceInterface {
 	/// Get memory reference.
 	fn memory(&self, index: ItemIndex) -> Result<Arc<MemoryInstance>, Error>;
 	/// Get global reference.
-	fn global<'a>(&self, index: ItemIndex, variable_type: Option<VariableType>, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>) -> Result<Arc<VariableInstance>, Error>;
+	fn global<'a>(&self, index: ItemIndex, variable_type: Option<VariableType>, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>) -> Result<Arc<VariableInstance>, Error>;
 	/// Get function type for given function index.
 	fn function_type(&self, function_index: ItemIndex) -> Result<FunctionSignature, Error>;
 	/// Get function type for given function index.
 	fn function_type_by_index(&self, type_index: u32) -> Result<FunctionSignature, Error>;
 	/// Get function reference.
-	fn function_reference<'a>(&self, index: ItemIndex, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>) -> Result<InternalFunctionReference<'a>, Error>;
+	fn function_reference<'a>(&self, index: ItemIndex, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>) -> Result<InternalFunctionReference<'a, E>, Error>;
 	/// Get function indirect reference.
-	fn function_reference_indirect<'a>(&self, table_idx: u32, type_idx: u32, func_idx: u32, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>) -> Result<InternalFunctionReference<'a>, Error>;
+	fn function_reference_indirect<'a>(&self, table_idx: u32, type_idx: u32, func_idx: u32, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>) -> Result<InternalFunctionReference<'a, E>, Error>;
 	/// Get internal function for interpretation.
 	fn function_body<'a>(&'a self, internal_index: u32) -> Result<Option<InternalFunction<'a>>, Error>;
 	/// Call function with given internal index.
-	fn call_internal_function(&self, outer: CallerContext, index: u32) -> Result<Option<RuntimeValue>, Error>;
+	fn call_internal_function(&self, outer: CallerContext<E>, index: u32) -> Result<Option<RuntimeValue>, InterpreterError<E>>;
 }
 
 /// Item index in items index space.
@@ -89,7 +89,7 @@ pub enum ItemIndex {
 }
 
 /// Module instance.
-pub struct ModuleInstance {
+pub struct ModuleInstance<E: CustomUserError> {
 	/// Module name.
 	name: String,
 	/// Module.
@@ -97,7 +97,7 @@ pub struct ModuleInstance {
 	/// Function labels.
 	functions_labels: HashMap<u32, HashMap<usize, usize>>,
 	/// Module imports.
-	imports: ModuleImports,
+	imports: ModuleImports<E>,
 	/// Module exports.
 	exports: HashMap<String, Vec<Internal>>,
 	/// Tables.
@@ -109,7 +109,7 @@ pub struct ModuleInstance {
 }
 
 /// Caller context.
-pub struct CallerContext<'a> {
+pub struct CallerContext<'a, E: 'a + CustomUserError> {
 	/// Value stack limit
 	pub value_stack_limit: usize,
 	/// Frame stack limit
@@ -117,19 +117,19 @@ pub struct CallerContext<'a> {
 	/// Stack of the input parameters
 	pub value_stack: &'a mut StackWithLimit<RuntimeValue>,
 	/// Execution-local external modules.
-	pub externals: &'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>,
+	pub externals: &'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>,
 }
 
 /// Internal function reference.
 #[derive(Clone)]
-pub struct InternalFunctionReference<'a> {
+pub struct InternalFunctionReference<'a, E: CustomUserError> {
 	/// Module reference.
-	pub module: Arc<ModuleInstanceInterface + 'a>,
+	pub module: Arc<ModuleInstanceInterface<E> + 'a>,
 	/// Internal function index.
 	pub internal_index: u32,
 }
 
-impl<'a> fmt::Debug for InternalFunctionReference<'a> {
+impl<'a, E> fmt::Debug for InternalFunctionReference<'a, E> where E: CustomUserError {
 	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 		write!(f, "InternalFunctionReference")
 	}
@@ -145,9 +145,9 @@ pub struct InternalFunction<'a> {
 	pub labels: &'a HashMap<usize, usize>,
 }
 
-impl<'a> ExecutionParams<'a> {
+impl<'a, E> ExecutionParams<'a, E> where E: CustomUserError {
 	/// Create new execution params with given externa; module override.
-	pub fn with_external(name: String, module: Arc<ModuleInstanceInterface + 'a>) -> Self {
+	pub fn with_external(name: String, module: Arc<ModuleInstanceInterface<E> + 'a>) -> Self {
 		let mut externals = HashMap::new();
 		externals.insert(name, module);
 		ExecutionParams {
@@ -163,8 +163,17 @@ impl<'a> ExecutionParams<'a> {
 	}
 }
 
-impl<'a> From<Vec<RuntimeValue>> for ExecutionParams<'a> {
-	fn from(args: Vec<RuntimeValue>) -> ExecutionParams<'a> {
+impl<'a, E> Default for ExecutionParams<'a, E> where E: CustomUserError {
+	fn default() -> Self {
+		ExecutionParams {
+			args: Vec::default(),
+			externals: HashMap::default(),
+		}
+	}
+}
+
+impl<'a, E> From<Vec<RuntimeValue>> for ExecutionParams<'a, E> where E: CustomUserError {
+	fn from(args: Vec<RuntimeValue>) -> ExecutionParams<'a, E> {
 		ExecutionParams {
 			args: args,
 			externals: HashMap::new(),
@@ -172,9 +181,9 @@ impl<'a> From<Vec<RuntimeValue>> for ExecutionParams<'a> {
 	}
 }
 
-impl ModuleInstance {
+impl<E> ModuleInstance<E> where E: CustomUserError {
 	/// Instantiate given module within program context.
-	pub fn new<'a>(program: Weak<ProgramInstanceEssence>, name: String, module: Module) -> Result<Self, Error> {
+	pub fn new<'a>(program: Weak<ProgramInstanceEssence<E>>, name: String, module: Module) -> Result<Self, Error> {
 		// load entries from import section
 		let imports = ModuleImports::new(program, module.import_section());
 
@@ -222,7 +231,7 @@ impl ModuleInstance {
 	}
 
 	/// Run instantiation-time procedures (validation). Module is not completely validated until this call.
-	pub fn instantiate<'a>(&mut self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>) -> Result<(), Error> {
+	pub fn instantiate<'a>(&mut self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>) -> Result<(), Error> {
 		// validate start section
 		if let Some(start_function) = self.module.start_section() {
 			let func_type_index = self.require_function(ItemIndex::IndexSpace(start_function))?;
@@ -388,7 +397,7 @@ impl ModuleInstance {
 	}
 
 	/// Run start function [if any].
-	pub fn run_start_function(&self) -> Result<(), Error> {
+	pub fn run_start_function(&self) -> Result<(), InterpreterError<E>> {
 		// execute start function (if any)
 		if let Some(start_function) = self.module.start_section() {
 			self.execute_index(start_function, ExecutionParams::default())?;
@@ -396,7 +405,7 @@ impl ModuleInstance {
 		Ok(())
 	}
 
-	fn self_ref<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>) -> Result<Arc<ModuleInstanceInterface + 'a>, Error> {
+	fn self_ref<'a>(&self, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>) -> Result<Arc<ModuleInstanceInterface<E> + 'a>, Error> {
 		self.imports.module(externals, &self.name)
 	}
 
@@ -420,8 +429,8 @@ impl ModuleInstance {
 	}
 }
 
-impl ModuleInstanceInterface for ModuleInstance {
-	fn execute_index(&self, index: u32, params: ExecutionParams) -> Result<Option<RuntimeValue>, Error> {
+impl<E> ModuleInstanceInterface<E> for ModuleInstance<E> where E: CustomUserError {
+	fn execute_index(&self, index: u32, params: ExecutionParams<E>) -> Result<Option<RuntimeValue>, InterpreterError<E>> {
 		let ExecutionParams { args, externals } = params;
 		let mut args = StackWithLimit::with_data(args, DEFAULT_VALUE_STACK_LIMIT);
 		let function_reference = self.function_reference(ItemIndex::IndexSpace(index), Some(&externals))?;
@@ -429,7 +438,7 @@ impl ModuleInstanceInterface for ModuleInstance {
 		function_reference.module.call_internal_function(function_context, function_reference.internal_index)
 	}
 
-	fn execute_export(&self, name: &str, params: ExecutionParams) -> Result<Option<RuntimeValue>, Error> {
+	fn execute_export(&self, name: &str, params: ExecutionParams<E>) -> Result<Option<RuntimeValue>, InterpreterError<E>> {
 		let index = self.exports.get(name)
 			.ok_or(Error::Function(format!("missing executable export with name {}", name)))
 			.and_then(|l| l.iter()
@@ -494,7 +503,7 @@ impl ModuleInstanceInterface for ModuleInstance {
 		}
 	}
 
-	fn global<'a>(&self, index: ItemIndex, variable_type: Option<VariableType>, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>) -> Result<Arc<VariableInstance>, Error> {
+	fn global<'a>(&self, index: ItemIndex, variable_type: Option<VariableType>, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>) -> Result<Arc<VariableInstance>, Error> {
 		match self.imports.parse_global_index(index) {
 			ItemIndex::IndexSpace(_) => unreachable!("parse_global_index resolves IndexSpace option"),
 			ItemIndex::Internal(index) => self.globals.get(index as usize).cloned()
@@ -533,7 +542,7 @@ impl ModuleInstanceInterface for ModuleInstance {
 			.map(FunctionSignature::Module)
 	}
 
-	fn function_reference<'a>(&self, index: ItemIndex, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>) -> Result<InternalFunctionReference<'a>, Error> {
+	fn function_reference<'a>(&self, index: ItemIndex, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>) -> Result<InternalFunctionReference<'a, E>, Error> {
 		match self.imports.parse_function_index(index) {
 			ItemIndex::IndexSpace(_) => unreachable!("parse_function_index resolves IndexSpace option"),
 			ItemIndex::Internal(index) => Ok(InternalFunctionReference {
@@ -555,7 +564,7 @@ impl ModuleInstanceInterface for ModuleInstance {
 		}
 	}
 
-	fn function_reference_indirect<'a>(&self, table_idx: u32, type_idx: u32, func_idx: u32, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>) -> Result<InternalFunctionReference<'a>, Error> {
+	fn function_reference_indirect<'a>(&self, table_idx: u32, type_idx: u32, func_idx: u32, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>) -> Result<InternalFunctionReference<'a, E>, Error> {
 		let table = self.table(ItemIndex::IndexSpace(table_idx))?;
 		let (module, index) = match table.get(func_idx)? {
 			RuntimeValue::AnyFunc(module, index) => (module.clone(), index),
@@ -591,7 +600,7 @@ impl ModuleInstanceInterface for ModuleInstance {
 		}))
 	}
 
-	fn call_internal_function(&self, mut outer: CallerContext, index: u32) -> Result<Option<RuntimeValue>, Error> {
+	fn call_internal_function(&self, mut outer: CallerContext<E>, index: u32) -> Result<Option<RuntimeValue>, InterpreterError<E>> {
 		let function_type = self.function_type(ItemIndex::Internal(index))?;
 		let args = prepare_function_args(&function_type, outer.value_stack)?;
 		let function_ref = InternalFunctionReference { module: self.self_ref(Some(outer.externals))?, internal_index: index };
@@ -600,9 +609,9 @@ impl ModuleInstanceInterface for ModuleInstance {
 	}
 }
 
-impl<'a> CallerContext<'a> {
+impl<'a, E> CallerContext<'a, E> where E: CustomUserError {
 	/// Top most args
-	pub fn topmost(args: &'a mut StackWithLimit<RuntimeValue>, externals: &'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>) -> Self {
+	pub fn topmost(args: &'a mut StackWithLimit<RuntimeValue>, externals: &'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>) -> Self {
 		CallerContext {
 			value_stack_limit: DEFAULT_VALUE_STACK_LIMIT,
 			frame_stack_limit: DEFAULT_FRAME_STACK_LIMIT,
@@ -612,7 +621,7 @@ impl<'a> CallerContext<'a> {
 	}
 
 	/// Nested context
-	pub fn nested(outer: &'a mut FunctionContext) -> Self {
+	pub fn nested(outer: &'a mut FunctionContext<E>) -> Self {
 		CallerContext {
 			value_stack_limit: outer.value_stack().limit() - outer.value_stack().len(),
 			frame_stack_limit: outer.frame_stack().limit() - outer.frame_stack().len(),
@@ -632,7 +641,7 @@ pub fn check_limits(limits: &ResizableLimits) -> Result<(), Error> {
 	Ok(())
 }
 
-fn get_initializer(expr: &InitExpr, module: &Module, imports: &ModuleImports, expected_type: VariableType) -> Result<RuntimeValue, Error> {
+fn get_initializer<E: CustomUserError>(expr: &InitExpr, module: &Module, imports: &ModuleImports<E>, expected_type: VariableType) -> Result<RuntimeValue, Error> {
 	let first_opcode = match expr.code().len() {
 		1 => &expr.code()[0],
 		2 if expr.code().len() == 2 && expr.code()[1] == Opcode::End => &expr.code()[0],
diff --git a/src/interpreter/program.rs b/src/interpreter/program.rs
index 42b89db..01375fe 100644
--- a/src/interpreter/program.rs
+++ b/src/interpreter/program.rs
@@ -2,23 +2,23 @@ use std::sync::Arc;
 use std::collections::HashMap;
 use parking_lot::RwLock;
 use elements::Module;
-use interpreter::Error;
+use interpreter::{Error, CustomUserError, InterpreterError};
 use interpreter::env::{self, env_module};
 use interpreter::module::{ModuleInstance, ModuleInstanceInterface};
 
 /// Program instance. Program is a set of instantiated modules.
-pub struct ProgramInstance {
+pub struct ProgramInstance<E: CustomUserError> {
 	/// Shared data reference.
-	essence: Arc<ProgramInstanceEssence>,
+	essence: Arc<ProgramInstanceEssence<E>>,
 }
 
 /// Program instance essence.
-pub struct ProgramInstanceEssence {
+pub struct ProgramInstanceEssence<E: CustomUserError> {
 	/// Loaded modules.
-	modules: RwLock<HashMap<String, Arc<ModuleInstanceInterface>>>,
+	modules: RwLock<HashMap<String, Arc<ModuleInstanceInterface<E>>>>,
 }
 
-impl ProgramInstance {
+impl<E> ProgramInstance<E> where E: CustomUserError {
 	/// Create new program instance.
 	pub fn new() -> Result<Self, Error> {
 		ProgramInstance::with_env_params(env::EnvParams::default())
@@ -32,7 +32,7 @@ impl ProgramInstance {
 	}
 
 	/// Instantiate module with validation.
-	pub fn add_module<'a>(&self, name: &str, module: Module, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>) -> Result<Arc<ModuleInstance>, Error> {
+	pub fn add_module<'a>(&self, name: &str, module: Module, externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>) -> Result<Arc<ModuleInstance<E>>, InterpreterError<E>> {
 		let mut module_instance = ModuleInstance::new(Arc::downgrade(&self.essence), name.into(), module)?;
 		module_instance.instantiate(externals)?;
 
@@ -43,19 +43,19 @@ impl ProgramInstance {
 	}
 
 	/// Insert instantiated module.
-	pub fn insert_loaded_module(&self, name: &str, module_instance: Arc<ModuleInstance>) -> Result<Arc<ModuleInstance>, Error> {
+	pub fn insert_loaded_module(&self, name: &str, module_instance: Arc<ModuleInstance<E>>) -> Result<Arc<ModuleInstance<E>>, Error> {
 		// replace existing module with the same name with new one
 		self.essence.modules.write().insert(name.into(), module_instance.clone());
 		Ok(module_instance)
 	}
 
 	/// Get one of the modules by name
-	pub fn module(&self, name: &str) -> Option<Arc<ModuleInstanceInterface>> {
+	pub fn module(&self, name: &str) -> Option<Arc<ModuleInstanceInterface<E>>> {
 		self.essence.module(name)
 	}
 }
 
-impl ProgramInstanceEssence {
+impl<E> ProgramInstanceEssence<E> where E: CustomUserError {
 	/// Create new program essence.
 	pub fn new() -> Result<Self, Error> {
 		ProgramInstanceEssence::with_env_params(env::EnvParams::default())
@@ -63,7 +63,7 @@ impl ProgramInstanceEssence {
 
 	pub fn with_env_params(env_params: env::EnvParams) -> Result<Self, Error> {
 		let mut modules = HashMap::new();
-		let env_module: Arc<ModuleInstanceInterface> = Arc::new(env_module(env_params)?);
+		let env_module: Arc<ModuleInstanceInterface<E>> = Arc::new(env_module(env_params)?);
 		modules.insert("env".into(), env_module);
 		Ok(ProgramInstanceEssence {
 			modules: RwLock::new(modules),
@@ -71,7 +71,7 @@ impl ProgramInstanceEssence {
 	}
 
 	/// Get module reference.
-	pub fn module(&self, name: &str) -> Option<Arc<ModuleInstanceInterface>> {
+	pub fn module(&self, name: &str) -> Option<Arc<ModuleInstanceInterface<E>>> {
 		self.modules.read().get(name).cloned()
 	}
 }
diff --git a/src/interpreter/runner.rs b/src/interpreter/runner.rs
index 906e48f..7e12c9d 100644
--- a/src/interpreter/runner.rs
+++ b/src/interpreter/runner.rs
@@ -6,7 +6,7 @@ use std::fmt::{self, Display};
 use std::iter::repeat;
 use std::collections::{HashMap, VecDeque};
 use elements::{Opcode, BlockType, Local};
-use interpreter::Error;
+use interpreter::{Error, CustomUserError, InterpreterError};
 use interpreter::module::{ModuleInstanceInterface, CallerContext, ItemIndex, InternalFunctionReference, FunctionSignature};
 use interpreter::stack::StackWithLimit;
 use interpreter::value::{
@@ -22,16 +22,18 @@ pub const DEFAULT_MEMORY_INDEX: u32 = 0;
 pub const DEFAULT_TABLE_INDEX: u32 = 0;
 
 /// Function interpreter.
-pub struct Interpreter;
+pub struct Interpreter<E: CustomUserError> {
+	_dummy: ::std::marker::PhantomData<E>,
+}
 
 /// Function execution context.
-pub struct FunctionContext<'a> {
+pub struct FunctionContext<'a, E: 'a + CustomUserError> {
 	/// Is context initialized.
 	pub is_initialized: bool,
 	/// Internal function reference.
-	pub function: InternalFunctionReference<'a>,
+	pub function: InternalFunctionReference<'a, E>,
 	/// Execution-local external modules.
-	pub externals: &'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>,
+	pub externals: &'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>,
 	/// Function return type.
 	pub return_type: BlockType,
 	/// Local variables.
@@ -46,13 +48,13 @@ pub struct FunctionContext<'a> {
 
 /// Interpreter action to execute after executing instruction.
 #[derive(Debug)]
-pub enum InstructionOutcome<'a> {
+pub enum InstructionOutcome<'a, E: CustomUserError> {
 	/// Continue with next instruction.
 	RunNextInstruction,
 	/// Branch to given frame.
 	Branch(usize),
 	/// Execute function call.
-	ExecuteCall(InternalFunctionReference<'a>),
+	ExecuteCall(InternalFunctionReference<'a, E>),
 	/// End current frame.
 	End,
 	/// Return from current function block.
@@ -60,15 +62,15 @@ pub enum InstructionOutcome<'a> {
 }
 
 /// Function run result.
-enum RunResult<'a> {
+enum RunResult<'a, E: 'a + CustomUserError> {
 	/// Function has returned (optional) value.
 	Return(Option<RuntimeValue>),
 	/// Function is calling other function.
-	NestedCall(FunctionContext<'a>),
+	NestedCall(FunctionContext<'a, E>),
 }
 
-impl Interpreter {
-	pub fn run_function(function_context: FunctionContext) -> Result<Option<RuntimeValue>, Error> {
+impl<E> Interpreter<E> where E: CustomUserError {
+	pub fn run_function(function_context: FunctionContext<E>) -> Result<Option<RuntimeValue>, InterpreterError<E>> {
 		let mut function_stack = VecDeque::new();
 		function_stack.push_back(function_context);
 
@@ -118,7 +120,7 @@ impl Interpreter {
 		}
 	}
 
-	fn do_run_function<'a>(function_context: &mut FunctionContext<'a>, function_body: &[Opcode], function_labels: &HashMap<usize, usize>) -> Result<RunResult<'a>, Error> {
+	fn do_run_function<'a>(function_context: &mut FunctionContext<'a, E>, function_body: &[Opcode], function_labels: &HashMap<usize, usize>) -> Result<RunResult<'a, E>, Error> {
 		loop {
 			let instruction = &function_body[function_context.position];
 
@@ -156,7 +158,7 @@ impl Interpreter {
 		}))
 	}
 
-	fn run_instruction<'a>(context: &mut FunctionContext<'a>, labels: &HashMap<usize, usize>, opcode: &Opcode) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_instruction<'a>(context: &mut FunctionContext<'a, E>, labels: &HashMap<usize, usize>, opcode: &Opcode) -> Result<InstructionOutcome<'a, E>, Error> {
 		match opcode {
 			&Opcode::Unreachable => Interpreter::run_unreachable(context),
 			&Opcode::Nop => Interpreter::run_nop(context),
@@ -350,25 +352,25 @@ impl Interpreter {
 		}
 	}
 
-	fn run_unreachable<'a>(_context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_unreachable<'a>(_context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error> {
 		Err(Error::Trap("programmatic".into()))
 	}
 
-	fn run_nop<'a>(_context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_nop<'a>(_context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error> {
 		Ok(InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_block<'a>(context: &mut FunctionContext<'a>, labels: &HashMap<usize, usize>, block_type: BlockType) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_block<'a>(context: &mut FunctionContext<'a, E>, labels: &HashMap<usize, usize>, block_type: BlockType) -> Result<InstructionOutcome<'a, E>, Error> {
 		context.push_frame(labels, BlockFrameType::Block, block_type)?;
 		Ok(InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_loop<'a>(context: &mut FunctionContext<'a>, labels: &HashMap<usize, usize>, block_type: BlockType) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_loop<'a>(context: &mut FunctionContext<'a, E>, labels: &HashMap<usize, usize>, block_type: BlockType) -> Result<InstructionOutcome<'a, E>, Error> {
 		context.push_frame(labels, BlockFrameType::Loop, block_type)?;
 		Ok(InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_if<'a>(context: &mut FunctionContext<'a>, labels: &HashMap<usize, usize>, block_type: BlockType) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_if<'a>(context: &mut FunctionContext<'a, E>, labels: &HashMap<usize, usize>, block_type: BlockType) -> Result<InstructionOutcome<'a, E>, Error> {
 		let branch = context.value_stack_mut().pop_as()?;
 		let block_frame_type = if branch { BlockFrameType::IfTrue } else {
 			let else_pos = labels[&context.position];
@@ -383,23 +385,23 @@ impl Interpreter {
 		context.push_frame(labels, block_frame_type, block_type).map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_else<'a>(context: &mut FunctionContext, labels: &HashMap<usize, usize>) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_else<'a>(context: &mut FunctionContext<E>, labels: &HashMap<usize, usize>) -> Result<InstructionOutcome<'a, E>, Error> {
 		let end_pos = labels[&context.position];
 		context.pop_frame(false)?;
 		context.position = end_pos;
 		Ok(InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_end<'a>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_end<'a>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error> {
 		context.pop_frame(false)?;
 		Ok(InstructionOutcome::End)
 	}
 
-	fn run_br<'a>(_context: &mut FunctionContext, label_idx: u32) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_br<'a>(_context: &mut FunctionContext<E>, label_idx: u32) -> Result<InstructionOutcome<'a, E>, Error> {
 		Ok(InstructionOutcome::Branch(label_idx as usize))
 	}
 
-	fn run_br_if<'a>(context: &mut FunctionContext, label_idx: u32) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_br_if<'a>(context: &mut FunctionContext<E>, label_idx: u32) -> Result<InstructionOutcome<'a, E>, Error> {
 		if context.value_stack_mut().pop_as()? {
 			Ok(InstructionOutcome::Branch(label_idx as usize))
 		} else {
@@ -407,20 +409,20 @@ impl Interpreter {
 		}
 	}
 
-	fn run_br_table<'a>(context: &mut FunctionContext, table: &Vec<u32>, default: u32) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_br_table<'a>(context: &mut FunctionContext<E>, table: &Vec<u32>, default: u32) -> Result<InstructionOutcome<'a, E>, Error> {
 		let index: u32 = context.value_stack_mut().pop_as()?;
 		Ok(InstructionOutcome::Branch(table.get(index as usize).cloned().unwrap_or(default) as usize))
 	}
 
-	fn run_return<'a>(_context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_return<'a>(_context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error> {
 		Ok(InstructionOutcome::Return)
 	}
 
-	fn run_call<'a>(context: &mut FunctionContext<'a>, func_idx: u32) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_call<'a>(context: &mut FunctionContext<'a, E>, func_idx: u32) -> Result<InstructionOutcome<'a, E>, Error> {
 		Ok(InstructionOutcome::ExecuteCall(context.module().function_reference(ItemIndex::IndexSpace(func_idx), Some(context.externals))?))
 	}
 
-	fn run_call_indirect<'a>(context: &mut FunctionContext<'a>, type_idx: u32) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_call_indirect<'a>(context: &mut FunctionContext<'a, E>, type_idx: u32) -> Result<InstructionOutcome<'a, E>, Error> {
 		let table_func_idx: u32 = context.value_stack_mut().pop_as()?;
 		let function_reference = context.module().function_reference_indirect(DEFAULT_TABLE_INDEX, type_idx, table_func_idx, Some(context.externals))?;
 		{
@@ -435,14 +437,14 @@ impl Interpreter {
 		Ok(InstructionOutcome::ExecuteCall(function_reference))
 	}
 
-	fn run_drop<'a>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_drop<'a>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error> {
 		context
 			.value_stack_mut()
 			.pop()
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_select<'a>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_select<'a>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error> {
 		context
 			.value_stack_mut()
 			.pop_triple()
@@ -457,32 +459,32 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_get_local<'a>(context: &mut FunctionContext, index: u32) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_get_local<'a>(context: &mut FunctionContext<E>, index: u32) -> Result<InstructionOutcome<'a, E>, Error> {
 		context.get_local(index as usize)
 			.map(|value| context.value_stack_mut().push(value))
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_set_local<'a>(context: &mut FunctionContext, index: u32) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_set_local<'a>(context: &mut FunctionContext<E>, index: u32) -> Result<InstructionOutcome<'a, E>, Error> {
 		let arg = context.value_stack_mut().pop()?;
 		context.set_local(index as usize, arg)
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_tee_local<'a>(context: &mut FunctionContext, index: u32) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_tee_local<'a>(context: &mut FunctionContext<E>, index: u32) -> Result<InstructionOutcome<'a, E>, Error> {
 		let arg = context.value_stack().top()?.clone();
 		context.set_local(index as usize, arg)
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_get_global<'a>(context: &mut FunctionContext, index: u32) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_get_global<'a>(context: &mut FunctionContext<E>, index: u32) -> Result<InstructionOutcome<'a, E>, Error> {
 		context.module()
 			.global(ItemIndex::IndexSpace(index), None, Some(context.externals))
 			.and_then(|g| context.value_stack_mut().push(g.get()))
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_set_global<'a>(context: &mut FunctionContext, index: u32) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_set_global<'a>(context: &mut FunctionContext<E>, index: u32) -> Result<InstructionOutcome<'a, E>, Error> {
 		context
 			.value_stack_mut()
 			.pop()
@@ -490,7 +492,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_load<'a, T>(context: &mut FunctionContext, _align: u32, offset: u32) -> Result<InstructionOutcome<'a>, Error>
+	fn run_load<'a, T>(context: &mut FunctionContext<E>, _align: u32, offset: u32) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T>, T: LittleEndianConvert {
 		let address = effective_address(offset, context.value_stack_mut().pop_as()?)?;
 		context.module()
@@ -501,7 +503,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_load_extend<'a, T, U>(context: &mut FunctionContext, _align: u32, offset: u32) -> Result<InstructionOutcome<'a>, Error>
+	fn run_load_extend<'a, T, U>(context: &mut FunctionContext<E>, _align: u32, offset: u32) -> Result<InstructionOutcome<'a, E>, Error>
 		where T: ExtendInto<U>, RuntimeValue: From<U>, T: LittleEndianConvert {
 		let address = effective_address(offset, context.value_stack_mut().pop_as()?)?;
 		let stack_value: U = context.module()
@@ -515,7 +517,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_store<'a, T>(context: &mut FunctionContext, _align: u32, offset: u32) -> Result<InstructionOutcome<'a>, Error>
+	fn run_store<'a, T>(context: &mut FunctionContext<E>, _align: u32, offset: u32) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: TryInto<T, Error>, T: LittleEndianConvert {
 		let stack_value = context
 			.value_stack_mut()
@@ -528,7 +530,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_store_wrap<'a, T, U>(context: &mut FunctionContext, _align: u32, offset: u32) -> Result<InstructionOutcome<'a>, Error>
+	fn run_store_wrap<'a, T, U>(context: &mut FunctionContext<E>, _align: u32, offset: u32) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: TryInto<T, Error>, T: WrapInto<U>, U: LittleEndianConvert {
 		let stack_value: T = context.value_stack_mut().pop().and_then(|v| v.try_into())?;
 		let stack_value = stack_value.wrap_into().into_little_endian();
@@ -539,7 +541,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_current_memory<'a>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_current_memory<'a>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error> {
 		context.module()
 			.memory(ItemIndex::IndexSpace(DEFAULT_MEMORY_INDEX))
 			.map(|m| m.size())
@@ -547,7 +549,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_grow_memory<'a>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_grow_memory<'a>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error> {
 		let pages: u32 = context.value_stack_mut().pop_as()?;
 		context.module()
 			.memory(ItemIndex::IndexSpace(DEFAULT_MEMORY_INDEX))
@@ -556,14 +558,14 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_const<'a>(context: &mut FunctionContext, val: RuntimeValue) -> Result<InstructionOutcome<'a>, Error> {
+	fn run_const<'a>(context: &mut FunctionContext<E>, val: RuntimeValue) -> Result<InstructionOutcome<'a, E>, Error> {
 		context
 			.value_stack_mut()
 			.push(val)
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_eqz<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_eqz<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: TryInto<T, Error>, T: PartialEq<T> + Default {
 		context
 			.value_stack_mut()
@@ -573,7 +575,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_eq<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_eq<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: TryInto<T, Error>, T: PartialEq<T> {
 		context
 			.value_stack_mut()
@@ -583,7 +585,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_ne<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_ne<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: TryInto<T, Error>, T: PartialEq<T> {
 		context
 			.value_stack_mut()
@@ -593,7 +595,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_lt<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_lt<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: TryInto<T, Error>, T: PartialOrd<T> + Display {
 		context
 			.value_stack_mut()
@@ -603,7 +605,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_gt<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_gt<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: TryInto<T, Error>, T: PartialOrd<T> {
 		context
 			.value_stack_mut()
@@ -613,7 +615,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_lte<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_lte<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: TryInto<T, Error>, T: PartialOrd<T> {
 		context
 			.value_stack_mut()
@@ -623,7 +625,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_gte<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_gte<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: TryInto<T, Error>, T: PartialOrd<T> {
 		context
 			.value_stack_mut()
@@ -633,7 +635,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_clz<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_clz<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Integer<T> {
 		context
 			.value_stack_mut()
@@ -643,7 +645,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_ctz<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_ctz<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Integer<T> {
 		context
 			.value_stack_mut()
@@ -653,7 +655,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_popcnt<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_popcnt<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Integer<T> {
 		context
 			.value_stack_mut()
@@ -663,7 +665,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_add<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_add<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: ArithmeticOps<T> {
 		context
 			.value_stack_mut()
@@ -673,7 +675,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_sub<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_sub<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: ArithmeticOps<T> {
 		context
 			.value_stack_mut()
@@ -683,7 +685,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_mul<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_mul<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: ArithmeticOps<T> {
 		context
 			.value_stack_mut()
@@ -693,7 +695,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_div<'a, T, U>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_div<'a, T, U>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: TransmuteInto<U> + Display, U: ArithmeticOps<U> + TransmuteInto<T> {
 		context
 			.value_stack_mut()
@@ -705,7 +707,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_rem<'a, T, U>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_rem<'a, T, U>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: TransmuteInto<U>, U: Integer<U> + TransmuteInto<T> {
 		context
 			.value_stack_mut()
@@ -717,7 +719,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_and<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_and<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<<T as ops::BitAnd>::Output> + TryInto<T, Error>, T: ops::BitAnd<T> {
 		context
 			.value_stack_mut()
@@ -727,7 +729,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_or<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_or<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<<T as ops::BitOr>::Output> + TryInto<T, Error>, T: ops::BitOr<T> {
 		context
 			.value_stack_mut()
@@ -737,7 +739,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_xor<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_xor<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<<T as ops::BitXor>::Output> + TryInto<T, Error>, T: ops::BitXor<T> {
 		context
 			.value_stack_mut()
@@ -747,7 +749,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_shl<'a, T>(context: &mut FunctionContext, mask: T) -> Result<InstructionOutcome<'a>, Error>
+	fn run_shl<'a, T>(context: &mut FunctionContext<E>, mask: T) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<<T as ops::Shl<T>>::Output> + TryInto<T, Error>, T: ops::Shl<T> + ops::BitAnd<T, Output=T> {
 		context
 			.value_stack_mut()
@@ -757,7 +759,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_shr<'a, T, U>(context: &mut FunctionContext, mask: U) -> Result<InstructionOutcome<'a>, Error>
+	fn run_shr<'a, T, U>(context: &mut FunctionContext<E>, mask: U) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: TransmuteInto<U>, U: ops::Shr<U> + ops::BitAnd<U, Output=U>, <U as ops::Shr<U>>::Output: TransmuteInto<T> {
 		context
 			.value_stack_mut()
@@ -769,7 +771,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_rotl<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_rotl<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Integer<T> {
 		context
 			.value_stack_mut()
@@ -779,7 +781,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_rotr<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_rotr<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Integer<T> {
 		context
 			.value_stack_mut()
@@ -789,7 +791,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_abs<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_abs<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Float<T> {
 		context
 			.value_stack_mut()
@@ -799,7 +801,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_neg<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_neg<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<<T as ops::Neg>::Output> + TryInto<T, Error>, T: ops::Neg {
 		context
 			.value_stack_mut()
@@ -809,7 +811,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_ceil<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_ceil<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Float<T> {
 		context
 			.value_stack_mut()
@@ -819,7 +821,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_floor<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_floor<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Float<T> {
 		context
 			.value_stack_mut()
@@ -829,7 +831,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_trunc<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_trunc<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Float<T> {
 		context
 			.value_stack_mut()
@@ -839,7 +841,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_nearest<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_nearest<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Float<T> {
 		context
 			.value_stack_mut()
@@ -849,7 +851,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_sqrt<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_sqrt<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Float<T> {
 		context
 			.value_stack_mut()
@@ -859,7 +861,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_min<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_min<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Float<T> {
 		context
 			.value_stack_mut()
@@ -869,7 +871,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_max<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_max<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Float<T> {
 		context
 			.value_stack_mut()
@@ -879,7 +881,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_copysign<'a, T>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_copysign<'a, T>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<T> + TryInto<T, Error>, T: Float<T> {
 		context
 			.value_stack_mut()
@@ -889,7 +891,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_wrap<'a, T, U>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_wrap<'a, T, U>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<U> + TryInto<T, Error>, T: WrapInto<U> {
 		context
 			.value_stack_mut()
@@ -899,7 +901,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_trunc_to_int<'a, T, U, V>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_trunc_to_int<'a, T, U, V>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<V> + TryInto<T, Error>, T: TryTruncateInto<U, Error>, U: TransmuteInto<V>,  {
 		context
 			.value_stack_mut()
@@ -910,7 +912,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_extend<'a, T, U, V>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_extend<'a, T, U, V>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<V> + TryInto<T, Error>, T: ExtendInto<U>, U: TransmuteInto<V> {
 		context
 			.value_stack_mut()
@@ -921,7 +923,7 @@ impl Interpreter {
 			.map(|_| InstructionOutcome::RunNextInstruction)
 	}
 
-	fn run_reinterpret<'a, T, U>(context: &mut FunctionContext) -> Result<InstructionOutcome<'a>, Error>
+	fn run_reinterpret<'a, T, U>(context: &mut FunctionContext<E>) -> Result<InstructionOutcome<'a, E>, Error>
 		where RuntimeValue: From<U>, RuntimeValue: TryInto<T, Error>, T: TransmuteInto<U> {
 		context
 			.value_stack_mut()
@@ -932,8 +934,8 @@ impl Interpreter {
 	}
 }
 
-impl<'a> FunctionContext<'a> {
-	pub fn new(function: InternalFunctionReference<'a>, externals: &'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>, value_stack_limit: usize, frame_stack_limit: usize, function_type: &FunctionSignature, args: Vec<VariableInstance>) -> Self {
+impl<'a, E> FunctionContext<'a, E> where E: CustomUserError {
+	pub fn new(function: InternalFunctionReference<'a, E>, externals: &'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>, value_stack_limit: usize, frame_stack_limit: usize, function_type: &FunctionSignature, args: Vec<VariableInstance>) -> Self {
 		FunctionContext {
 			is_initialized: false,
 			function: function,
@@ -946,7 +948,7 @@ impl<'a> FunctionContext<'a> {
 		}
 	}
 
-	pub fn nested(&mut self, function: InternalFunctionReference<'a>) -> Result<Self, Error> {
+	pub fn nested(&mut self, function: InternalFunctionReference<'a, E>) -> Result<Self, Error> {
 		let (function_locals, function_return_type) = {
 			let function_type = function.module.function_type(ItemIndex::Internal(function.internal_index))?;
 			let function_return_type = function_type.return_type().map(|vt| BlockType::Value(vt)).unwrap_or(BlockType::NoResult);
@@ -982,15 +984,15 @@ impl<'a> FunctionContext<'a> {
 		Ok(())
 	}
 
-	pub fn module(&self) -> &Arc<ModuleInstanceInterface + 'a> {
+	pub fn module(&self) -> &Arc<ModuleInstanceInterface<E> + 'a> {
 		&self.function.module
 	}
 
-	pub fn externals(&self) -> &HashMap<String, Arc<ModuleInstanceInterface + 'a>> {
+	pub fn externals(&self) -> &HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>> {
 		&self.externals
 	}
 
-	pub fn set_local(&mut self, index: usize, value: RuntimeValue) -> Result<InstructionOutcome<'a>, Error> {
+	pub fn set_local(&mut self, index: usize, value: RuntimeValue) -> Result<InstructionOutcome<'a, E>, Error> {
 		self.locals.get_mut(index)
 			.ok_or(Error::Local(format!("expected to have local with index {}", index)))
 			.and_then(|l| l.set(value))
@@ -1072,7 +1074,7 @@ impl<'a> FunctionContext<'a> {
 	}
 }
 
-impl<'a> fmt::Debug for FunctionContext<'a> {
+impl<'a, E> fmt::Debug for FunctionContext<'a, E> where E: CustomUserError {
 	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 		write!(f, "FunctionContext")
 	}
diff --git a/src/interpreter/tests/basics.rs b/src/interpreter/tests/basics.rs
index 30f79c4..b474ea2 100644
--- a/src/interpreter/tests/basics.rs
+++ b/src/interpreter/tests/basics.rs
@@ -5,11 +5,10 @@ use std::collections::HashMap;
 use builder::module;
 use elements::{ExportEntry, Internal, ImportEntry, External, GlobalEntry, GlobalType,
 	InitExpr, ValueType, BlockType, Opcodes, Opcode, FunctionType};
-use interpreter::Error;
+use interpreter::{Error, CustomUserError, DummyCustomUserError, InterpreterError, ProgramInstance, ModuleInstance, CustomProgramInstance};
 use interpreter::env_native::{env_native_module, UserDefinedElements, UserFunctionExecutor, UserFunctionDescriptor};
 use interpreter::memory::MemoryInstance;
-use interpreter::module::{ModuleInstance, ModuleInstanceInterface, CallerContext, ItemIndex, ExecutionParams, ExportEntryType, FunctionSignature};
-use interpreter::program::ProgramInstance;
+use interpreter::module::{ModuleInstanceInterface, CallerContext, ItemIndex, ExecutionParams, ExportEntryType, FunctionSignature};
 use interpreter::validator::{FunctionValidationContext, Validator};
 use interpreter::value::{RuntimeValue, TryInto};
 use interpreter::variable::{VariableInstance, ExternalVariableValue, VariableType};
@@ -115,6 +114,11 @@ const SIGNATURES: &'static [UserFunctionDescriptor] = &[
 		SIGNATURE_I32_I32,
 		Some(ValueType::I32),
 	),
+	UserFunctionDescriptor::Static(
+		"err",
+		SIGNATURE_I32_I32,
+		Some(ValueType::I32),
+	),
 ];
 
 const NO_SIGNATURES: &'static [UserFunctionDescriptor] = &[];
@@ -135,14 +139,28 @@ impl ExternalVariableValue for MeasuredVariable {
 	}
 }
 
+// custom user error
+#[derive(Debug, Clone, PartialEq)]
+struct UserError {
+	error_code: i32,
+}
+
+impl ::std::fmt::Display for UserError {
+	fn fmt(&self, f: &mut ::std::fmt::Formatter) -> Result<(), ::std::fmt::Error> {
+		write!(f, "{}", self.error_code)
+	}
+}
+
+impl CustomUserError for UserError {}
+
 // user function executor
 struct FunctionExecutor {
 	pub memory: Arc<MemoryInstance>,
 	pub values: Vec<i32>,
 }
 
-impl UserFunctionExecutor for FunctionExecutor {
-	fn execute(&mut self, name: &str, context: CallerContext) -> Result<Option<RuntimeValue>, Error> {
+impl UserFunctionExecutor<UserError> for FunctionExecutor {
+	fn execute(&mut self, name: &str, context: CallerContext<UserError>) -> Result<Option<RuntimeValue>, InterpreterError<UserError>> {
 		match name {
 			"add" => {
 				let memory_value = self.memory.get(0, 1).unwrap()[0];
@@ -166,7 +184,10 @@ impl UserFunctionExecutor for FunctionExecutor {
 				self.values.push(diff as i32);
 				Ok(Some(RuntimeValue::I32(diff as i32)))
 			},
-			_ => Err(Error::Trap("not implemented".into())),
+			"err" => {
+				Err(InterpreterError::User(UserError { error_code: 777 }))
+			},
+			_ => Err(Error::Trap("not implemented".into()).into()),
 		}
 	}
 }
@@ -174,7 +195,7 @@ impl UserFunctionExecutor for FunctionExecutor {
 #[test]
 fn native_env_function() {
 	// create new program
-	let program = ProgramInstance::new().unwrap();
+	let program = CustomProgramInstance::new().unwrap();
 	// => env module is created
 	let env_instance = program.module("env").unwrap();
 	// => linear memory is created
@@ -186,7 +207,7 @@ fn native_env_function() {
 		values: Vec::new(),
 	};
 	{
-		let functions: UserDefinedElements = UserDefinedElements {
+		let functions = UserDefinedElements {
 			executor: Some(&mut executor),
 			globals: HashMap::new(),
 			functions: ::std::borrow::Cow::from(SIGNATURES),
@@ -282,6 +303,40 @@ fn native_env_global() {
 	}
 }
 
+#[test]
+fn native_custom_error() {
+	let program = CustomProgramInstance::new().unwrap();
+	let env_instance = program.module("env").unwrap();
+	let env_memory = env_instance.memory(ItemIndex::Internal(0)).unwrap();
+	let mut executor = FunctionExecutor { memory: env_memory.clone(), values: Vec::new() };
+	let functions = UserDefinedElements {
+		executor: Some(&mut executor),
+		globals: HashMap::new(),
+		functions: ::std::borrow::Cow::from(SIGNATURES),
+	};
+	let native_env_instance = Arc::new(env_native_module(env_instance, functions).unwrap());
+	let params = ExecutionParams::with_external("env".into(), native_env_instance);
+
+	let module = module()
+		.with_import(ImportEntry::new("env".into(), "err".into(), External::Function(0)))
+		.function()
+			.signature().param().i32().param().i32().return_type().i32().build()
+			.body().with_opcodes(Opcodes::new(vec![
+				Opcode::GetLocal(0),
+				Opcode::GetLocal(1),
+				Opcode::Call(0),
+				Opcode::End,
+			])).build()
+			.build()
+		.build();
+
+	let module_instance = program.add_module("main", module, Some(&params.externals)).unwrap();
+	assert_eq!(module_instance.execute_index(0, params.clone().add_argument(RuntimeValue::I32(7)).add_argument(RuntimeValue::I32(0))),
+		Err(InterpreterError::User(UserError { error_code: 777 })));
+	assert_eq!(module_instance.execute_index(1, params.clone().add_argument(RuntimeValue::I32(7)).add_argument(RuntimeValue::I32(0))),
+		Err(InterpreterError::User(UserError { error_code: 777 })));
+}
+
 #[test]
 fn import_env_mutable_global() {
 	let program = ProgramInstance::new().unwrap();
@@ -295,7 +350,7 @@ fn import_env_mutable_global() {
 
 #[test]
 fn env_native_export_entry_type_check() {
-	let program = ProgramInstance::new().unwrap();
+	let program = CustomProgramInstance::new().unwrap();
 	let mut function_executor = FunctionExecutor {
 		memory: program.module("env").unwrap().memory(ItemIndex::Internal(0)).unwrap(),
 		values: Vec::new(),
diff --git a/src/interpreter/tests/wabt.rs b/src/interpreter/tests/wabt.rs
index 1244c74..576f49c 100644
--- a/src/interpreter/tests/wabt.rs
+++ b/src/interpreter/tests/wabt.rs
@@ -3,9 +3,7 @@
 use std::sync::Arc;
 use builder::module;
 use elements::{ValueType, Opcodes, Opcode, BlockType, Local};
-use interpreter::Error;
-use interpreter::module::{ModuleInstanceInterface, ItemIndex};
-use interpreter::program::ProgramInstance;
+use interpreter::{Error, InterpreterError, ProgramInstance, ModuleInstanceInterface, CustomModuleInstanceInterface, ItemIndex};
 use interpreter::value::{RuntimeValue, TryInto};
 
 fn make_function_i32(body: Opcodes) -> (ProgramInstance, Arc<ModuleInstanceInterface>) {
@@ -28,6 +26,10 @@ fn run_function_i32(module: &Arc<ModuleInstanceInterface>, arg: i32) -> Result<i
 	module
 		.execute_index(0, vec![RuntimeValue::I32(arg)].into())
 		.map(|r| r.unwrap().try_into().unwrap())
+		.map_err(|e| match e {
+			InterpreterError::Internal(e) => e,
+			_ => unreachable!(),
+		})
 }
 
 /// https://github.com/WebAssembly/wabt/blob/8e1f6031e9889ba770c7be4a9b084da5f14456a0/test/interp/unreachable.txt
@@ -733,9 +735,9 @@ fn callindirect_2() {
 	assert_eq!(module.execute_index(3, vec![RuntimeValue::I32(10), RuntimeValue::I32(4), RuntimeValue::I32(0)].into()).unwrap().unwrap(), RuntimeValue::I32(14));
 	assert_eq!(module.execute_index(3, vec![RuntimeValue::I32(10), RuntimeValue::I32(4), RuntimeValue::I32(1)].into()).unwrap().unwrap(), RuntimeValue::I32(6));
 	assert_eq!(module.execute_index(3, vec![RuntimeValue::I32(10), RuntimeValue::I32(4), RuntimeValue::I32(2)].into()).unwrap_err(),
-		Error::Function("expected indirect function with signature ([I32, I32]) -> Some(I32) when got with ([I32]) -> Some(I32)".into()));
+		InterpreterError::Internal(Error::Function("expected indirect function with signature ([I32, I32]) -> Some(I32) when got with ([I32]) -> Some(I32)".into())));
 	assert_eq!(module.execute_index(3, vec![RuntimeValue::I32(10), RuntimeValue::I32(4), RuntimeValue::I32(3)].into()).unwrap_err(),
-		Error::Table("trying to read table item with index 3 when there are only 3 items".into()));
+		InterpreterError::Internal(Error::Table("trying to read table item with index 3 when there are only 3 items".into())));
 }
 
 /// https://github.com/WebAssembly/wabt/blob/8e1f6031e9889ba770c7be4a9b084da5f14456a0/test/interp/select.txt
diff --git a/src/interpreter/tests/wasm.rs b/src/interpreter/tests/wasm.rs
index a005726..a5f9219 100644
--- a/src/interpreter/tests/wasm.rs
+++ b/src/interpreter/tests/wasm.rs
@@ -1,9 +1,7 @@
 use elements::deserialize_file;
 use elements::Module;
-use interpreter::EnvParams;
-use interpreter::ExecutionParams;
+use interpreter::{EnvParams, ExecutionParams, ProgramInstance};
 use interpreter::module::ModuleInstanceInterface;
-use interpreter::program::ProgramInstance;
 use interpreter::value::RuntimeValue;
 
 // Name of function contained in WASM file (note the leading underline)
diff --git a/src/interpreter/validator.rs b/src/interpreter/validator.rs
index 43f411b..bef362d 100644
--- a/src/interpreter/validator.rs
+++ b/src/interpreter/validator.rs
@@ -2,7 +2,7 @@ use std::u32;
 use std::sync::Arc;
 use std::collections::HashMap;
 use elements::{Opcode, BlockType, ValueType};
-use interpreter::Error;
+use interpreter::{Error, CustomUserError};
 use interpreter::runner::{DEFAULT_MEMORY_INDEX, DEFAULT_TABLE_INDEX};
 use interpreter::module::{ModuleInstance, ModuleInstanceInterface, ItemIndex, FunctionSignature};
 use interpreter::stack::StackWithLimit;
@@ -12,11 +12,11 @@ use interpreter::variable::VariableType;
 const NATURAL_ALIGNMENT: u32 = 0xFFFFFFFF;
 
 /// Function validation context.
-pub struct FunctionValidationContext<'a> {
+pub struct FunctionValidationContext<'a, E: 'a + CustomUserError> {
 	/// Wasm module instance (in process of instantiation).
-	module_instance: &'a ModuleInstance,
+	module_instance: &'a ModuleInstance<E>,
 	/// Native externals.
-	externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>,
+	externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>,
 	/// Current instruction position.
 	position: usize,
 	/// Local variables.
@@ -75,7 +75,9 @@ pub enum BlockFrameType {
 }
 
 /// Function validator.
-pub struct Validator;
+pub struct Validator<E: CustomUserError> {
+	_dummy: ::std::marker::PhantomData<E>,
+}
 
 /// Instruction outcome.
 #[derive(Debug, Clone)]
@@ -86,8 +88,8 @@ pub enum InstructionOutcome {
 	Unreachable,
 }
 
-impl Validator {
-	pub fn validate_function(context: &mut FunctionValidationContext, block_type: BlockType, body: &[Opcode]) -> Result<(), Error> {
+impl<E> Validator<E> where E: CustomUserError {
+	pub fn validate_function(context: &mut FunctionValidationContext<E>, block_type: BlockType, body: &[Opcode]) -> Result<(), Error> {
 		context.push_label(BlockFrameType::Function, block_type)?;
 		Validator::validate_function_block(context, body)?;
 		while !context.frame_stack.is_empty() {
@@ -97,7 +99,7 @@ impl Validator {
 		Ok(())
 	}
 
-	fn validate_function_block(context: &mut FunctionValidationContext, body: &[Opcode]) -> Result<(), Error> {
+	fn validate_function_block(context: &mut FunctionValidationContext<E>, body: &[Opcode]) -> Result<(), Error> {
 		let body_len = body.len();
 		if body_len == 0 {
 			return Err(Error::Validation("Non-empty function body expected".into()));
@@ -117,7 +119,7 @@ impl Validator {
 		}
 	}
 
-	fn validate_instruction(context: &mut FunctionValidationContext, opcode: &Opcode) -> Result<InstructionOutcome, Error> {
+	fn validate_instruction(context: &mut FunctionValidationContext<E>, opcode: &Opcode) -> Result<InstructionOutcome, Error> {
 		debug!(target: "validator", "validating {:?}", opcode);
 		match opcode {
 			&Opcode::Unreachable => Ok(InstructionOutcome::Unreachable),
@@ -312,49 +314,49 @@ impl Validator {
 		}
 	}
 
-	fn validate_const(context: &mut FunctionValidationContext, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
+	fn validate_const(context: &mut FunctionValidationContext<E>, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
 		context.push_value(value_type)?;
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_unop(context: &mut FunctionValidationContext, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
+	fn validate_unop(context: &mut FunctionValidationContext<E>, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
 		context.pop_value(value_type)?;
 		context.push_value(value_type)?;
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_binop(context: &mut FunctionValidationContext, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
+	fn validate_binop(context: &mut FunctionValidationContext<E>, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
 		context.pop_value(value_type)?;
 		context.pop_value(value_type)?;
 		context.push_value(value_type)?;
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_testop(context: &mut FunctionValidationContext, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
+	fn validate_testop(context: &mut FunctionValidationContext<E>, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
 		context.pop_value(value_type)?;
 		context.push_value(ValueType::I32.into())?;
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_relop(context: &mut FunctionValidationContext, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
+	fn validate_relop(context: &mut FunctionValidationContext<E>, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
 		context.pop_value(value_type)?;
 		context.pop_value(value_type)?;
 		context.push_value(ValueType::I32.into())?;
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_cvtop(context: &mut FunctionValidationContext, value_type1: StackValueType, value_type2: StackValueType) -> Result<InstructionOutcome, Error> {
+	fn validate_cvtop(context: &mut FunctionValidationContext<E>, value_type1: StackValueType, value_type2: StackValueType) -> Result<InstructionOutcome, Error> {
 		context.pop_value(value_type1)?;
 		context.push_value(value_type2)?;
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_drop(context: &mut FunctionValidationContext) -> Result<InstructionOutcome, Error> {
+	fn validate_drop(context: &mut FunctionValidationContext<E>) -> Result<InstructionOutcome, Error> {
 		context.pop_any_value().map(|_| ())?;
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_select(context: &mut FunctionValidationContext) -> Result<InstructionOutcome, Error> {
+	fn validate_select(context: &mut FunctionValidationContext<E>) -> Result<InstructionOutcome, Error> {
 		context.pop_value(ValueType::I32.into())?;
 		let select_type = context.pop_any_value()?;
 		context.pop_value(select_type)?;
@@ -362,13 +364,13 @@ impl Validator {
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_get_local(context: &mut FunctionValidationContext, index: u32) -> Result<InstructionOutcome, Error> {
+	fn validate_get_local(context: &mut FunctionValidationContext<E>, index: u32) -> Result<InstructionOutcome, Error> {
 		let local_type = context.require_local(index)?;
 		context.push_value(local_type)?;
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_set_local(context: &mut FunctionValidationContext, index: u32) -> Result<InstructionOutcome, Error> {
+	fn validate_set_local(context: &mut FunctionValidationContext<E>, index: u32) -> Result<InstructionOutcome, Error> {
 		let local_type = context.require_local(index)?;
 		let value_type = context.pop_any_value()?;
 		if local_type != value_type {
@@ -377,7 +379,7 @@ impl Validator {
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_tee_local(context: &mut FunctionValidationContext, index: u32) -> Result<InstructionOutcome, Error> {
+	fn validate_tee_local(context: &mut FunctionValidationContext<E>, index: u32) -> Result<InstructionOutcome, Error> {
 		let local_type = context.require_local(index)?;
 		let value_type = context.tee_any_value()?;
 		if local_type != value_type {
@@ -386,13 +388,13 @@ impl Validator {
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_get_global(context: &mut FunctionValidationContext, index: u32) -> Result<InstructionOutcome, Error> {
+	fn validate_get_global(context: &mut FunctionValidationContext<E>, index: u32) -> Result<InstructionOutcome, Error> {
 		let global_type = context.require_global(index, None)?;
 		context.push_value(global_type)?;
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_set_global(context: &mut FunctionValidationContext, index: u32) -> Result<InstructionOutcome, Error> {
+	fn validate_set_global(context: &mut FunctionValidationContext<E>, index: u32) -> Result<InstructionOutcome, Error> {
 		let global_type = context.require_global(index, Some(true))?;
 		let value_type = context.pop_any_value()?;
 		if global_type != value_type {
@@ -401,7 +403,7 @@ impl Validator {
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_load(context: &mut FunctionValidationContext, align: u32, max_align: u32, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
+	fn validate_load(context: &mut FunctionValidationContext<E>, align: u32, max_align: u32, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
 		if align != NATURAL_ALIGNMENT {
 			if 1u32.checked_shl(align).unwrap_or(u32::MAX) > max_align {
 				return Err(Error::Validation(format!("Too large memory alignment 2^{} (expected at most {})", align, max_align)));
@@ -414,7 +416,7 @@ impl Validator {
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_store(context: &mut FunctionValidationContext, align: u32, max_align: u32, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
+	fn validate_store(context: &mut FunctionValidationContext<E>, align: u32, max_align: u32, value_type: StackValueType) -> Result<InstructionOutcome, Error> {
 		if align != NATURAL_ALIGNMENT {
 			if 1u32.checked_shl(align).unwrap_or(u32::MAX) > max_align {
 				return Err(Error::Validation(format!("Too large memory alignment 2^{} (expected at most {})", align, max_align)));
@@ -427,20 +429,20 @@ impl Validator {
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_block(context: &mut FunctionValidationContext, block_type: BlockType) -> Result<InstructionOutcome, Error> {
+	fn validate_block(context: &mut FunctionValidationContext<E>, block_type: BlockType) -> Result<InstructionOutcome, Error> {
 		context.push_label(BlockFrameType::Block, block_type).map(|_| InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_loop(context: &mut FunctionValidationContext, block_type: BlockType) -> Result<InstructionOutcome, Error> {
+	fn validate_loop(context: &mut FunctionValidationContext<E>, block_type: BlockType) -> Result<InstructionOutcome, Error> {
 		context.push_label(BlockFrameType::Loop, block_type).map(|_| InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_if(context: &mut FunctionValidationContext, block_type: BlockType) -> Result<InstructionOutcome, Error> {
+	fn validate_if(context: &mut FunctionValidationContext<E>, block_type: BlockType) -> Result<InstructionOutcome, Error> {
 		context.pop_value(ValueType::I32.into())?;
 		context.push_label(BlockFrameType::IfTrue, block_type).map(|_| InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_else(context: &mut FunctionValidationContext) -> Result<InstructionOutcome, Error> {
+	fn validate_else(context: &mut FunctionValidationContext<E>) -> Result<InstructionOutcome, Error> {
 		let block_type = {
 			let top_frame = context.top_label()?;
 			if top_frame.frame_type != BlockFrameType::IfTrue {
@@ -456,7 +458,7 @@ impl Validator {
 		context.push_label(BlockFrameType::IfFalse, block_type).map(|_| InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_end(context: &mut FunctionValidationContext) -> Result<InstructionOutcome, Error> {
+	fn validate_end(context: &mut FunctionValidationContext<E>) -> Result<InstructionOutcome, Error> {
 		{
 			let top_frame = context.top_label()?;
 			if top_frame.frame_type == BlockFrameType::IfTrue {
@@ -469,7 +471,7 @@ impl Validator {
 		context.pop_label().map(|_| InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_br(context: &mut FunctionValidationContext, idx: u32) -> Result<InstructionOutcome, Error> {
+	fn validate_br(context: &mut FunctionValidationContext<E>, idx: u32) -> Result<InstructionOutcome, Error> {
 		let (frame_type, frame_block_type) = {
 			let frame = context.require_label(idx)?;
 			(frame.frame_type, frame.block_type)
@@ -483,7 +485,7 @@ impl Validator {
 		Ok(InstructionOutcome::Unreachable)
 	}
 
-	fn validate_br_if(context: &mut FunctionValidationContext, idx: u32) -> Result<InstructionOutcome, Error> {
+	fn validate_br_if(context: &mut FunctionValidationContext<E>, idx: u32) -> Result<InstructionOutcome, Error> {
 		context.pop_value(ValueType::I32.into())?;
 		if let BlockType::Value(value_type) = context.require_label(idx)?.block_type {
 			context.tee_value(value_type.into())?;
@@ -491,7 +493,7 @@ impl Validator {
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_br_table(context: &mut FunctionValidationContext, table: &Vec<u32>, default: u32) -> Result<InstructionOutcome, Error> {
+	fn validate_br_table(context: &mut FunctionValidationContext<E>, table: &Vec<u32>, default: u32) -> Result<InstructionOutcome, Error> {
 		let mut required_block_type = None;
 		
 		{
@@ -523,14 +525,14 @@ impl Validator {
 		Ok(InstructionOutcome::Unreachable)
 	}
 
-	fn validate_return(context: &mut FunctionValidationContext) -> Result<InstructionOutcome, Error> {
+	fn validate_return(context: &mut FunctionValidationContext<E>) -> Result<InstructionOutcome, Error> {
 		if let BlockType::Value(value_type) = context.return_type()? {
 			context.tee_value(value_type.into())?;
 		}
 		Ok(InstructionOutcome::Unreachable)
 	}
 
-	fn validate_call(context: &mut FunctionValidationContext, idx: u32) -> Result<InstructionOutcome, Error> {
+	fn validate_call(context: &mut FunctionValidationContext<E>, idx: u32) -> Result<InstructionOutcome, Error> {
 		let (argument_types, return_type) = context.require_function(idx)?;
 		for argument_type in argument_types.iter().rev() {
 			context.pop_value((*argument_type).into())?;
@@ -541,7 +543,7 @@ impl Validator {
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_call_indirect(context: &mut FunctionValidationContext, idx: u32) -> Result<InstructionOutcome, Error> {
+	fn validate_call_indirect(context: &mut FunctionValidationContext<E>, idx: u32) -> Result<InstructionOutcome, Error> {
 		context.require_table(DEFAULT_TABLE_INDEX, VariableType::AnyFunc)?;
 
 		context.pop_value(ValueType::I32.into())?;
@@ -555,13 +557,13 @@ impl Validator {
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_current_memory(context: &mut FunctionValidationContext) -> Result<InstructionOutcome, Error> {
+	fn validate_current_memory(context: &mut FunctionValidationContext<E>) -> Result<InstructionOutcome, Error> {
 		context.require_memory(DEFAULT_MEMORY_INDEX)?;
 		context.push_value(ValueType::I32.into())?;
 		Ok(InstructionOutcome::ValidateNextInstruction)
 	}
 
-	fn validate_grow_memory(context: &mut FunctionValidationContext) -> Result<InstructionOutcome, Error> {
+	fn validate_grow_memory(context: &mut FunctionValidationContext<E>) -> Result<InstructionOutcome, Error> {
 		context.require_memory(DEFAULT_MEMORY_INDEX)?;
 		context.pop_value(ValueType::I32.into())?;
 		context.push_value(ValueType::I32.into())?;
@@ -569,10 +571,10 @@ impl Validator {
 	}
 }
 
-impl<'a> FunctionValidationContext<'a> {
+impl<'a, E> FunctionValidationContext<'a, E> where E: CustomUserError {
 	pub fn new(
-		module_instance: &'a ModuleInstance, 
-		externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface + 'a>>>,
+		module_instance: &'a ModuleInstance<E>, 
+		externals: Option<&'a HashMap<String, Arc<ModuleInstanceInterface<E> + 'a>>>,
 		locals: &'a [ValueType], 
 		value_stack_limit: usize, 
 		frame_stack_limit: usize, 
diff --git a/src/lib.rs b/src/lib.rs
index 680de78..b6f5751 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -21,7 +21,10 @@ pub use elements::{
 
 pub use interpreter::{
     ProgramInstance,
+    CustomProgramInstance,
     ModuleInstance,
+    CustomModuleInstance,
     ModuleInstanceInterface,
+    CustomModuleInstanceInterface,
     RuntimeValue,
 };