diff --git a/include/boost/detail/call_traits.hpp b/include/boost/detail/call_traits.hpp
index 4753f7e..0b1cfa3 100644
--- a/include/boost/detail/call_traits.hpp
+++ b/include/boost/detail/call_traits.hpp
@@ -34,7 +34,7 @@ namespace boost{
 
 namespace detail{
 
-template <typename T, bool small>
+template <typename T, bool small_>
 struct ct_imp2
 {
    typedef const T& param_type;