diff --git a/include/boost/fusion/adapted/mpl/detail/empty_impl.hpp b/include/boost/fusion/adapted/mpl/detail/empty_impl.hpp
index 14e8dfa2..7c62c94b 100644
--- a/include/boost/fusion/adapted/mpl/detail/empty_impl.hpp
+++ b/include/boost/fusion/adapted/mpl/detail/empty_impl.hpp
@@ -2,7 +2,7 @@
     Copyright (c) 2001-2006 Joel de Guzman
     Copyright (c) 2005-2006 Dan Marsden
 
-    Distributed under the Boost Software License, Version 1.0. (See accompanying 
+    Distributed under the Boost Software License, Version 1.0. (See accompanying
     file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 ==============================================================================*/
 #if !defined(BOOST_FUSION_EMPTY_IMPL_31122005_1554)
@@ -10,12 +10,15 @@
 
 #include <boost/mpl/empty.hpp>
 
-namespace boost { namespace fusion 
+namespace boost { namespace fusion
 {
     struct mpl_sequence_tag;
 
     namespace extension
     {
+        template <typename Sequence>
+        struct empty_impl;
+
         template <>
         struct empty_impl<mpl_sequence_tag>
         {