diff -r 11404088d1a5 -r 3fc2a801c39e lemon/concepts/maps.h
--- a/lemon/concepts/maps.h	Fri Sep 25 12:24:16 2009 +0200
+++ b/lemon/concepts/maps.h	Sat Sep 26 07:08:10 2009 +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;