diff --git a/derive/src/error.rs b/derive/src/error.rs
index 3895f51..209b8c6 100644
--- a/derive/src/error.rs
+++ b/derive/src/error.rs
@@ -1,4 +1,4 @@
-use proc_macro2::{TokenStream, Span};
+use proc_macro2::{Span, TokenStream};
 use quote::{quote_spanned, ToTokens};
 
 macro_rules! err_span {
@@ -21,10 +21,7 @@ impl CompileError {
     }
 
     pub fn new(msg: String) -> Self {
-        CompileError {
-            span: None,
-            msg,
-        }
+        CompileError { span: None, msg }
     }
 }
 
@@ -34,6 +31,7 @@ impl ToTokens for CompileError {
         let span = self.span.unwrap_or_else(|| Span::call_site());
         (quote_spanned! { span=>
             compile_error!(#msg);
-        }).to_tokens(dst);
+        })
+        .to_tokens(dst);
     }
 }
diff --git a/derive/src/lib.rs b/derive/src/lib.rs
index d9b4aff..71932e6 100644
--- a/derive/src/lib.rs
+++ b/derive/src/lib.rs
@@ -24,7 +24,7 @@
 //!
 //! #[derive_externals]
 //! impl<'a> NonStaticExternals<'a> {
-//!     pub fn hello(&self, a: u32, b: u32) -> u32 {
+//!     pub fn add(&self, a: u32, b: u32) -> u32 {
 //!         a + b
 //!     }
 //!
@@ -60,8 +60,6 @@ pub fn derive_externals(_attr: TokenStream, input: TokenStream) -> TokenStream {
             codegen::codegen(&ext_def, &mut input);
             input.into()
         }
-        Err(err) => {
-            (quote::quote! { #err }).into()
-        }
+        Err(err) => (quote::quote! { #err }).into(),
     }
 }
diff --git a/derive/src/parser.rs b/derive/src/parser.rs
index 8e42ccb..ec954c4 100644
--- a/derive/src/parser.rs
+++ b/derive/src/parser.rs
@@ -37,7 +37,8 @@ pub struct ImplBlockDef {
 
 /// Parse an incoming stream of tokens into externalities definition.
 pub fn parse(input: proc_macro2::TokenStream) -> Result<ImplBlockDef, CompileError> {
-    let item_impl = syn::parse2::<syn::ItemImpl>(input).map_err(|_| CompileError::new("failed to parse".to_string()))?;
+    let item_impl = syn::parse2::<syn::ItemImpl>(input)
+        .map_err(|_| CompileError::new("failed to parse".to_string()))?;
 
     let mut funcs = vec![];
 
@@ -57,9 +58,12 @@ pub fn parse(input: proc_macro2::TokenStream) -> Result<ImplBlockDef, CompileErr
                         if idx == 0 {
                             match input {
                                 FnArg::SelfRef(_) => return None,
-                                _ => return Some(
-                                    Err(err_span!(input.span(), "only &self and &mut self supported as first argument"))
-                                ),
+                                _ => {
+                                    return Some(Err(err_span!(
+                                        input.span(),
+                                        "only &self and &mut self supported as first argument"
+                                    )));
+                                }
                             }
                         }
 
diff --git a/wasmi/tests/derives.rs b/wasmi/tests/derives.rs
index 2f05f24..1f8eb9c 100644
--- a/wasmi/tests/derives.rs
+++ b/wasmi/tests/derives.rs
@@ -2,6 +2,8 @@
 
 extern crate wasmi;
 extern crate wasmi_derive;
+#[macro_use]
+extern crate assert_matches;
 
 use std::fmt;
 use wasmi::HostError;
@@ -22,7 +24,7 @@ struct NonStaticExternals<'a> {
 
 #[derive_externals]
 impl<'a> NonStaticExternals<'a> {
-    pub fn hello(&self, a: u32, b: u32) -> u32 {
+    pub fn add(&self, a: u32, b: u32) -> u32 {
         a + b
     }
 
@@ -34,3 +36,117 @@ impl<'a> NonStaticExternals<'a> {
         Err(NoInfoError)
     }
 }
+
+mod tests {
+    extern crate wabt;
+
+    use super::*;
+    use wasmi::{ImportsBuilder, Module, ModuleInstance, RuntimeValue};
+
+    macro_rules! gen_test {
+        ($test_name:ident, $wat:expr, |$instance:ident| $verify:expr) => {
+            #[test]
+            fn $test_name() {
+                // We don't test wat compiliation, loading or decoding.
+                let wasm = &wabt::wat2wasm($wat).expect("invalid wat");
+                let module = Module::from_buffer(&wasm).expect("can't load module");
+
+                let resolver = NonStaticExternals::resolver();
+
+                let mut imports = ImportsBuilder::new();
+                imports.push_resolver("env", &resolver);
+                let $instance = ModuleInstance::new(&module, &imports);
+
+                $verify
+            }
+        };
+    }
+
+    gen_test! { it_works,
+        r#"
+        (module
+            (import "env" "add" (func $add (param i32 i32) (result i32)))
+            (import "env" "increment" (func $increment))
+            (import "env" "traps" (func $traps))
+
+            (export "add" (func $add))
+            (export "increment" (func $increment))
+            (export "traps" (func $traps))
+        )
+        "#,
+        |not_started_instance_result| {
+            let mut state = 0;
+            let mut externals = NonStaticExternals {
+                state: &mut state,
+            };
+
+            let instance = not_started_instance_result.unwrap().assert_no_start();
+            assert_matches!(
+                instance.invoke_export(
+                    "traps",
+                    &[],
+                    &mut externals,
+                ),
+                Err(_)
+            );
+
+            assert_eq!(*externals.state, 0);
+            assert_matches!(
+                instance.invoke_export(
+                    "increment",
+                    &[],
+                    &mut externals,
+                ),
+                Ok(None)
+            );
+            assert_eq!(*externals.state, 1);
+
+            assert_matches!(
+                instance.invoke_export(
+                    "add",
+                    &[RuntimeValue::I32(5), RuntimeValue::I32(2)],
+                    &mut externals,
+                ),
+                Ok(Some(RuntimeValue::I32(7)))
+            );
+        }
+    }
+
+    gen_test! { wrong_signature,
+        r#"
+        (module
+            (import "env" "add" (func $add (param i64 i32) (result i32)))
+        )
+        "#,
+        |result| {
+            match result {
+                Ok(_) => panic!(),
+                Err(err) => {
+                    assert_eq!(
+                        &format!("{:?}", err),
+                        r#"Instantiation("Export add has different signature Signature { params: [I64, I32], return_type: Some(I32) }")"#,
+                    );
+                }
+            }
+        }
+    }
+
+    gen_test! { nonexistent_name,
+        r#"
+        (module
+            (import "env" "foo" (func $foo))
+        )
+        "#,
+        |result| {
+            match result {
+                Ok(_) => panic!(),
+                Err(err) => {
+                    assert_eq!(
+                        &format!("{:?}", err),
+                        r#"Instantiation("Export foo not found")"#,
+                    );
+                }
+            }
+        }
+    }
+}