diff --git a/src/interpreter/mod.rs b/src/interpreter/mod.rs
index 2471561..c85390b 100644
--- a/src/interpreter/mod.rs
+++ b/src/interpreter/mod.rs
@@ -1,11 +1,35 @@
 //! WebAssembly interpreter module.
 
+use std::any::TypeId;
+
 /// Custom user error.
-pub trait UserError: 'static + ::std::fmt::Display + ::std::fmt::Debug + Clone + PartialEq {
+pub trait UserError: 'static + ::std::fmt::Display + ::std::fmt::Debug {
+	#[doc(hidden)]
+    fn __private_get_type_id__(&self) -> TypeId {
+        TypeId::of::<Self>()
+    }
+}
+
+impl UserError {
+	pub fn downcast_ref<T: UserError>(&self) -> Option<&T> {
+        if self.__private_get_type_id__() == TypeId::of::<T>() {
+            unsafe { Some(&*(self as *const UserError as *const T)) }
+        } else {
+            None
+        }
+    }
+
+	pub fn downcast_mut<T: UserError>(&mut self) -> Option<&mut T> {
+		if self.__private_get_type_id__() == TypeId::of::<T>() {
+            unsafe { Some(&mut *(self as *mut UserError as *mut T)) }
+        } else {
+            None
+        }
+    }
 }
 
 /// Internal interpreter error.
-#[derive(Debug, Clone, PartialEq)]
+#[derive(Debug)]
 pub enum Error<E> where E: UserError {
 	/// Program-level error.
 	Program(String),
@@ -38,7 +62,8 @@ pub enum Error<E> where E: UserError {
 	/// Trap.
 	Trap(String),
 	/// Custom user error.
-	User(E),
+	User(Box<UserError>),
+	Other(E),
 }
 
 impl<E> Into<String> for Error<E> where E: UserError {
@@ -60,6 +85,7 @@ impl<E> Into<String> for Error<E> where E: UserError {
 			Error::Native(s) => s,
 			Error::Trap(s) => format!("trap: {}", s),
 			Error::User(e) => format!("user: {}", e),
+			Error::Other(_) => panic!("TODO: Remove this arm "),
 		}
 	}
 }
@@ -83,6 +109,7 @@ impl<E> ::std::fmt::Display for Error<E> where E: UserError {
 			Error::Native(ref s) => write!(f, "Native: {}", s),
 			Error::Trap(ref s) => write!(f, "Trap: {}", s),
 			Error::User(ref e) => write!(f, "User: {}", e),
+			Error::Other(_) => panic!("TODO: Remove this arm "),
 		}
 	}
 }
@@ -99,7 +126,7 @@ impl ::std::fmt::Display for DummyUserError {
 
 impl<U> From<U> for Error<U> where U: UserError + Sized {
 	fn from(e: U) -> Self {
-		Error::User(e)
+		Error::User(Box::new(e))
 	}
 }
 
diff --git a/src/interpreter/module.rs b/src/interpreter/module.rs
index 69f641a..f3aa332 100644
--- a/src/interpreter/module.rs
+++ b/src/interpreter/module.rs
@@ -121,7 +121,6 @@ pub struct CallerContext<'a, E: 'a + UserError> {
 }
 
 /// Internal function reference.
-#[derive(Clone)]
 pub struct InternalFunctionReference<'a, E: UserError> {
 	/// Module reference.
 	pub module: Arc<ModuleInstanceInterface<E> + 'a>,
@@ -129,6 +128,16 @@ pub struct InternalFunctionReference<'a, E: UserError> {
 	pub internal_index: u32,
 }
 
+// TODO: This impl should be removed once `E` not needed anymore.
+impl<'a, E> Clone for InternalFunctionReference<'a, E> where E: UserError {
+	fn clone(&self) -> InternalFunctionReference<'a, E> {
+		InternalFunctionReference {
+			module: Arc::clone(&self.module),
+			internal_index: self.internal_index,
+		}
+	}
+}
+
 impl<'a, E> fmt::Debug for InternalFunctionReference<'a, E> where E: UserError {
 	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 		write!(f, "InternalFunctionReference")
diff --git a/src/interpreter/tests/basics.rs b/src/interpreter/tests/basics.rs
index bca315e..7235e5f 100644
--- a/src/interpreter/tests/basics.rs
+++ b/src/interpreter/tests/basics.rs
@@ -186,7 +186,7 @@ impl UserFunctionExecutor<UserErrorWithCode> for FunctionExecutor {
 				Ok(Some(RuntimeValue::I32(diff as i32)))
 			},
 			"err" => {
-				Err(Error::User(UserErrorWithCode { error_code: 777 }))
+				Err(Error::User(Box::new(UserErrorWithCode { error_code: 777 })))
 			},
 			_ => Err(Error::Trap("not implemented".into()).into()),
 		}
@@ -401,10 +401,29 @@ fn native_custom_error() {
 		.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(Error::User(UserErrorWithCode { error_code: 777 })));
-	assert_eq!(module_instance.execute_index(1, params.clone().add_argument(RuntimeValue::I32(7)).add_argument(RuntimeValue::I32(0))),
-		Err(Error::User(UserErrorWithCode { error_code: 777 })));
+	let user_error1 = match module_instance.execute_index(
+		0,
+		params
+			.clone()
+			.add_argument(RuntimeValue::I32(7))
+			.add_argument(RuntimeValue::I32(0)),
+	) {
+		Err(Error::User(user_error)) => user_error,
+		result => panic!("Unexpected result {:?}", result),
+	};
+	assert_eq!(user_error1.downcast_ref::<UserErrorWithCode>().unwrap(), &UserErrorWithCode { error_code: 777 });
+
+	let user_error2 = match module_instance.execute_index(
+		0,
+		params
+			.clone()
+			.add_argument(RuntimeValue::I32(7))
+			.add_argument(RuntimeValue::I32(0)),
+	) {
+		Err(Error::User(user_error)) => user_error,
+		result => panic!("Unexpected result {:?}", result),
+	};
+	assert_eq!(user_error2.downcast_ref::<UserErrorWithCode>().unwrap(), &UserErrorWithCode { error_code: 777 });
 }
 
 #[test]
@@ -434,21 +453,21 @@ fn env_native_export_entry_type_check() {
 	assert!(native_env_instance.export_entry("add", &ExportEntryType::Function(FunctionSignature::Module(&FunctionType::new(vec![ValueType::I32, ValueType::I32], Some(ValueType::I32))))).is_ok());
 	match native_env_instance.export_entry("add", &ExportEntryType::Function(FunctionSignature::Module(&FunctionType::new(vec![], Some(ValueType::I32))))) {
 		Err(Error::Validation(_)) => { },
-		result => panic!("Unexpected result {:?}.", result),
+		result => panic!("Unexpected result {:?}", result),
 	}
 	match native_env_instance.export_entry("add", &ExportEntryType::Function(FunctionSignature::Module(&FunctionType::new(vec![ValueType::I32, ValueType::I32], None)))) {
 		Err(Error::Validation(_)) => { },
-		result => panic!("Unexpected result {:?}.", result),
+		result => panic!("Unexpected result {:?}", result),
 	}
 	match native_env_instance.export_entry("add", &ExportEntryType::Function(FunctionSignature::Module(&FunctionType::new(vec![ValueType::I32, ValueType::I32], Some(ValueType::I64))))) {
 		Err(Error::Validation(_)) => { },
-		result => panic!("Unexpected result {:?}.", result),
+		result => panic!("Unexpected result {:?}", result),
 	}
 
 	assert!(native_env_instance.export_entry("ext_global", &ExportEntryType::Global(VariableType::I32)).is_ok());
 	match native_env_instance.export_entry("ext_global", &ExportEntryType::Global(VariableType::F32)) {
 		Err(Error::Validation(_)) => { },
-		result => panic!("Unexpected result {:?}.", result),
+		result => panic!("Unexpected result {:?}", result),
 	}
 }
 
diff --git a/src/interpreter/tests/wabt.rs b/src/interpreter/tests/wabt.rs
index 636a7d5..bbf07d5 100644
--- a/src/interpreter/tests/wabt.rs
+++ b/src/interpreter/tests/wabt.rs
@@ -35,7 +35,12 @@ fn unreachable() {
 		Opcode::Unreachable,	// trap
 		Opcode::End]));
 
-	assert_eq!(run_function_i32(&module, 0).unwrap_err(), Error::Trap("programmatic".into()));
+	match run_function_i32(&module, 0) {
+		Err(Error::Trap(msg)) => {
+			assert_eq!(msg, "programmatic");
+		},
+		result => panic!("Unexpected result {:?}", result),
+	}
 }
 
 #[test]
@@ -730,10 +735,24 @@ fn callindirect_2() {
 	let module = program.add_module("main", module, None).unwrap();
 	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()));
-	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()));
+	match module.execute_index(3, vec![RuntimeValue::I32(10), RuntimeValue::I32(4), RuntimeValue::I32(2)].into()) {
+		Err(Error::Function(msg)) => {
+			assert_eq!(
+				&msg,
+				"expected indirect function with signature ([I32, I32]) -> Some(I32) when got with ([I32]) -> Some(I32)"
+			);
+		}
+		result => panic!("Unexpected result {:?}", result),
+	}
+	match module.execute_index(3, vec![RuntimeValue::I32(10), RuntimeValue::I32(4), RuntimeValue::I32(3)].into()) {
+		Err(Error::Table(msg)) => {
+			assert_eq!(
+				&msg,
+				"trying to read table item with index 3 when there are only 3 items"
+			)
+		},
+		result => panic!("Unexpected result {:?}", result),
+	}
 }
 
 /// https://github.com/WebAssembly/wabt/blob/8e1f6031e9889ba770c7be4a9b084da5f14456a0/test/interp/select.txt