diff --git a/include/boost/thread/locks.hpp b/include/boost/thread/locks.hpp
index e2a9e6eb..0fe66bb6 100644
--- a/include/boost/thread/locks.hpp
+++ b/include/boost/thread/locks.hpp
@@ -629,7 +629,10 @@ namespace boost
             using base::mutex;
             using base::release;
             typedef typename base::bool_type bool_type;
-            using base::operator bool_type;
+            operator bool_type() const
+            {
+                return static_cast<base const&>(*this);
+            }
         };
         
         template<typename MutexType1,typename MutexType2>