diff -r 58c330ad0b5c -r 6be1f9bd2ac0 lemon/concepts/maps.h
--- a/lemon/concepts/maps.h	Sun Oct 04 10:15:32 2009 +0200
+++ b/lemon/concepts/maps.h	Wed Dec 09 11:14:06 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;