diff --git a/lemon/bits/map_extender.h b/lemon/bits/map_extender.h
--- a/lemon/bits/map_extender.h
+++ b/lemon/bits/map_extender.h
@@ -49,6 +49,8 @@
     typedef typename Parent::Reference Reference;
     typedef typename Parent::ConstReference ConstReference;
 
+    typedef typename Parent::ReferenceMapTag ReferenceMapTag;
+
     class MapIt;
     class ConstMapIt;
 
@@ -191,6 +193,8 @@
     typedef typename Parent::Reference Reference;
     typedef typename Parent::ConstReference ConstReference;
 
+    typedef typename Parent::ReferenceMapTag ReferenceMapTag;
+
     class MapIt;
     class ConstMapIt;
 
diff --git a/lemon/concepts/maps.h b/lemon/concepts/maps.h
--- a/lemon/concepts/maps.h
+++ b/lemon/concepts/maps.h
@@ -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;