diff -r 66156a3498ea -r b4af20d02ae0 lemon/concepts/maps.h
--- a/lemon/concepts/maps.h	Mon Mar 14 08:56:54 2011 +0100
+++ b/lemon/concepts/maps.h	Fri Apr 15 09:26:09 2011 +0200
@@ -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;