diff --git a/lemon/concepts/maps.h b/lemon/concepts/maps.h
--- a/lemon/concepts/maps.h
+++ b/lemon/concepts/maps.h
@@ -68,6 +68,7 @@
         const Key& key;
         const typename _ReadMap::Key& own_key;
         const _ReadMap& m;
+        Constraints() {}
       };
 
     };
@@ -109,6 +110,7 @@
         const typename _WriteMap::Key& own_key;
         const typename _WriteMap::Value& own_val;
         _WriteMap& m;
+        Constraints() {}
       };
     };
 
@@ -129,7 +131,8 @@
 
       /// Returns the value associated with the given key.
       Value operator[](const Key &) const {
-        return *static_cast<Value *>(0);
+        Value *r = 0;
+        return *r;
       }
 
       /// Sets the value associated with the given key.
@@ -169,12 +172,14 @@
 
       /// Returns a reference to the value associated with the given key.
       Reference operator[](const Key &) {
-        return *static_cast<Value *>(0);
+        Value *r = 0;
+        return *r;
       }
 
       /// Returns a const reference to the value associated with the given key.
       ConstReference operator[](const Key &) const {
-        return *static_cast<Value *>(0);
+        Value *r = 0;
+        return *r;
       }
 
       /// Sets the value associated with the given key.
@@ -205,6 +210,7 @@
         typename _ReferenceMap::Reference own_ref;
         typename _ReferenceMap::ConstReference own_cref;
         _ReferenceMap& m;
+        Constraints() {}
       };
     };