diff --git a/include/boost/smart_ptr/detail/sp_convertible.hpp b/include/boost/smart_ptr/detail/sp_convertible.hpp
index fe44069..eb39797 100644
--- a/include/boost/smart_ptr/detail/sp_convertible.hpp
+++ b/include/boost/smart_ptr/detail/sp_convertible.hpp
@@ -45,7 +45,7 @@ template< class Y, class T > struct sp_convertible
     static yes f( T* );
     static no  f( ... );
 
-    enum _vt { value = sizeof( f( static_cast<Y*>(0) ) ) == sizeof(yes) };
+    enum _vt { value = sizeof( (f)( static_cast<Y*>(0) ) ) == sizeof(yes) };
 };
 
 struct sp_empty