diff -r c160bf9f18ef -r 580af8cf2f6a lemon/concepts/maps.h
--- a/lemon/concepts/maps.h	Thu Nov 05 10:01:02 2009 +0100
+++ b/lemon/concepts/maps.h	Thu Nov 05 10:23:16 2009 +0100
@@ -182,7 +182,8 @@
 
       template<typename _ReferenceMap>
       struct Constraints {
-        void constraints() {
+        typename enable_if<typename _ReferenceMap::ReferenceMapTag, void>::type
+        constraints() {
           checkConcept<ReadWriteMap<K, T>, _ReferenceMap >();
           ref = m[key];
           m[key] = val;