diff --git a/examples/tictactoe.rs b/examples/tictactoe.rs
index b0bd551..7e8d66c 100644
--- a/examples/tictactoe.rs
+++ b/examples/tictactoe.rs
@@ -180,18 +180,6 @@ impl<'a> Externals for Runtime<'a> {
 			_ => panic!("unknown function index")
 		}
 	}
-
-	fn check_signature(&self, index: usize, sig: &FunctionType) -> bool {
-		match index {
-			SET_FUNC_INDEX => {
-				sig.params() == &[ValueType::I32] && sig.return_type() == None
-			}
-			GET_FUNC_INDEX => {
-				sig.params() == &[ValueType::I32] && sig.return_type() == Some(ValueType::I32)
-			}
-			_ => panic!("unknown function index")
-		}
-	}
 }
 
 struct RuntimeModuleImportResolver;
diff --git a/spec/src/run.rs b/spec/src/run.rs
index dafe5d0..ba68df3 100644
--- a/spec/src/run.rs
+++ b/spec/src/run.rs
@@ -81,13 +81,6 @@ impl Externals for SpecModule {
             _ => panic!("SpecModule doesn't provide function at index {}", index),
         }
     }
-
-    fn check_signature(&self, index: usize, _signature: &FunctionType) -> bool {
-        match index {
-            PRINT_FUNC_INDEX => true,
-            _ => false,
-        }
-    }
 }
 
 impl ModuleImportResolver for SpecModule {
diff --git a/src/interpreter/host.rs b/src/interpreter/host.rs
index 5d1ffab..d77dc1c 100644
--- a/src/interpreter/host.rs
+++ b/src/interpreter/host.rs
@@ -1,5 +1,4 @@
 use std::any::TypeId;
-use elements::FunctionType;
 use interpreter::value::RuntimeValue;
 use interpreter::Error;
 
@@ -38,8 +37,6 @@ pub trait Externals {
 		index: usize,
 		args: &[RuntimeValue],
 	) -> Result<Option<RuntimeValue>, Error>;
-
-	fn check_signature(&self, index: usize, signature: &FunctionType) -> bool;
 }
 
 pub struct NopExternals;
@@ -52,8 +49,4 @@ impl Externals for NopExternals {
 	) -> Result<Option<RuntimeValue>, Error> {
 		Err(Error::Trap("invoke index on no-op externals".into()))
 	}
-
-	fn check_signature(&self, _index: usize, _signature: &FunctionType) -> bool {
-		false
-	}
 }
diff --git a/src/interpreter/tests/host.rs b/src/interpreter/tests/host.rs
index d8bf61b..204063a 100644
--- a/src/interpreter/tests/host.rs
+++ b/src/interpreter/tests/host.rs
@@ -141,7 +141,9 @@ impl Externals for TestHost {
 			_ => panic!("env doesn't provide function at index {}", index),
 		}
 	}
+}
 
+impl TestHost {
 	fn check_signature(&self, index: usize, func_type: &FunctionType) -> bool {
 		if index == RECURSE_FUNC_INDEX {
 			// This function requires special handling because it is polymorphic.
@@ -477,10 +479,6 @@ fn defer_providing_externals() {
 				_ => panic!("env module doesn't provide function at index {}", index),
 			}
 		}
-
-		fn check_signature(&self, _index: usize, func_type: &FunctionType) -> bool {
-			func_type.params() == &[ValueType::I32] && func_type.return_type() == None
-		}
 	}
 
 	let module = parse_wat(