diff --git a/include/boost/mpl/aux_/push_back_impl.hpp b/include/boost/mpl/aux_/push_back_impl.hpp
index 3ece711..27e7a60 100644
--- a/include/boost/mpl/aux_/push_back_impl.hpp
+++ b/include/boost/mpl/aux_/push_back_impl.hpp
@@ -25,7 +25,7 @@
 
 namespace boost { namespace mpl {
 
-struct has_push_back_arg;
+struct has_push_back_arg {};
 
 // agurt 05/feb/04: no default implementation; the stub definition is needed 
 // to enable the default 'has_push_back' implementation below
diff --git a/include/boost/mpl/aux_/push_front_impl.hpp b/include/boost/mpl/aux_/push_front_impl.hpp
index 4010532..5b83ee7 100644
--- a/include/boost/mpl/aux_/push_front_impl.hpp
+++ b/include/boost/mpl/aux_/push_front_impl.hpp
@@ -25,7 +25,7 @@
 
 namespace boost { namespace mpl {
 
-struct has_push_front_arg;
+struct has_push_front_arg {};
 
 // agurt 05/feb/04: no default implementation; the stub definition is needed 
 // to enable the default 'has_push_front' implementation below