diff --git a/examples/tictactoe.rs b/examples/tictactoe.rs
index ef8f210..9a68b33 100644
--- a/examples/tictactoe.rs
+++ b/examples/tictactoe.rs
@@ -7,6 +7,7 @@ use parity_wasm::interpreter::{
 	Error as InterpreterError, ModuleInstance, UserError, ModuleRef,
 	HostFuncIndex, Externals, RuntimeValue, GlobalInstance, TableInstance, MemoryInstance,
 	TableRef, MemoryRef, GlobalRef, FuncRef, TryInto, ImportResolver, FuncInstance,
+	HostGlobalIndex, HostMemoryIndex, HostTableIndex,
 };
 use parity_wasm::elements::{Error as DeserializationError};
 use parity_wasm::ValidationError;
@@ -191,15 +192,15 @@ impl<'a> Externals for Runtime<'a> {
 		}
 	}
 
-	fn memory_by_index(&self, _index: usize) -> &MemoryInstance {
+	fn memory_by_index(&self, _index: HostMemoryIndex) -> Option<&MemoryInstance> {
 		panic!("host module doesn't export any memories")
 	}
 
-	fn table_by_index(&self, _index: usize) -> &TableInstance {
+	fn table_by_index(&self, _index: HostTableIndex) -> Option<&TableInstance> {
 		panic!("host module doesn't export any tables")
 	}
 
-	fn global_by_index(&self, _index: usize) -> &GlobalInstance {
+	fn global_by_index(&self, _index: HostGlobalIndex) -> Option<&GlobalInstance> {
 		panic!("host module doesn't export any globals")
 	}
 }
diff --git a/src/interpreter/host.rs b/src/interpreter/host.rs
index 20b8c55..a2c33fc 100644
--- a/src/interpreter/host.rs
+++ b/src/interpreter/host.rs
@@ -6,6 +6,9 @@ use interpreter::value::RuntimeValue;
 use interpreter::Error;
 
 pub type HostFuncIndex = u32;
+pub type HostMemoryIndex = u32;
+pub type HostTableIndex = u32;
+pub type HostGlobalIndex = u32;
 
 pub trait Externals {
 	fn invoke_index(
@@ -16,9 +19,9 @@ pub trait Externals {
 
 	fn check_signature(&self, index: HostFuncIndex, signature: &FunctionType) -> bool;
 
-	fn memory_by_index(&self, index: usize) -> &MemoryInstance;
-	fn table_by_index(&self, index: usize) -> &TableInstance;
-	fn global_by_index(&self, index: usize) -> &GlobalInstance;
+	fn memory_by_index(&self, index: HostMemoryIndex) -> Option<&MemoryInstance>;
+	fn table_by_index(&self, index: HostTableIndex) -> Option<&TableInstance>;
+	fn global_by_index(&self, index: HostGlobalIndex) -> Option<&GlobalInstance>;
 }
 
 pub struct EmptyExternals;
@@ -29,22 +32,22 @@ impl Externals for EmptyExternals {
 		_index: HostFuncIndex,
 		_args: &[RuntimeValue],
 	) -> Result<Option<RuntimeValue>, Error> {
-		panic!("called invoke_index on EmptyExternals")
+		Err(Error::Trap("invoke index on empty externals".into()))
 	}
 
 	fn check_signature(&self, _index: HostFuncIndex, _signature: &FunctionType) -> bool {
-		panic!("called check_signature on EmptyExternals")
+		false
 	}
 
-	fn memory_by_index(&self, _index: usize) -> &MemoryInstance {
-		panic!("called memory_by_index on EmptyExternals")
+	fn memory_by_index(&self, _index: HostMemoryIndex) -> Option<&MemoryInstance> {
+		None
 	}
 
-	fn table_by_index(&self, _index: usize) -> &TableInstance {
-		panic!("called table_by_index on EmptyExternals")
+	fn table_by_index(&self, _index: HostTableIndex) -> Option<&TableInstance> {
+		None
 	}
 
-	fn global_by_index(&self, _index: usize) -> &GlobalInstance {
-		panic!("called global_by_index on EmptyExternals")
+	fn global_by_index(&self, _index: HostGlobalIndex) -> Option<&GlobalInstance> {
+		None
 	}
 }
diff --git a/src/interpreter/mod.rs b/src/interpreter/mod.rs
index ca3d3ea..ef8523c 100644
--- a/src/interpreter/mod.rs
+++ b/src/interpreter/mod.rs
@@ -153,7 +153,7 @@ pub use self::memory::{MemoryInstance, MemoryRef};
 pub use self::table::{TableInstance, TableRef};
 pub use self::program::ProgramInstance;
 pub use self::value::{RuntimeValue, TryInto};
-pub use self::host::{Externals, HostFuncIndex, EmptyExternals};
+pub use self::host::{Externals, HostFuncIndex, HostGlobalIndex, HostMemoryIndex, HostTableIndex, EmptyExternals};
 pub use self::imports::{ImportResolver, Imports};
 pub use self::module::{ModuleInstance, ModuleRef};
 pub use self::global::{GlobalInstance, GlobalRef};