diff --git a/src/libraries/turtle/expectation.hpp b/src/libraries/turtle/expectation.hpp
index 6eae1de..34f1be2 100644
--- a/src/libraries/turtle/expectation.hpp
+++ b/src/libraries/turtle/expectation.hpp
@@ -38,6 +38,12 @@ namespace mock
         typedef BOOST_DEDUCED_TYPENAME
             boost::function< Signature >::result_type result_type;
 
+        template< typename Args >
+        struct sig
+        {
+            typedef result_type type;
+        };
+
     private:
         typedef detail::matcher< result_type,
                                  Signature,