diff --git a/include/boost/multi_index/detail/value_compare.hpp b/include/boost/multi_index/detail/value_compare.hpp
index 18513c3..a55046d 100644
--- a/include/boost/multi_index/detail/value_compare.hpp
+++ b/include/boost/multi_index/detail/value_compare.hpp
@@ -33,7 +33,7 @@ struct value_comparison:std::binary_function<Value,Value,bool>
 
   bool operator()(
     typename call_traits<Value>::param_type x,
-    typename call_traits<Value>::param_type y)
+    typename call_traits<Value>::param_type y)const
   {
     return comp(key(x),key(y));
   }