diff --git a/lemon/bits/traits.h b/lemon/bits/traits.h
--- a/lemon/bits/traits.h
+++ b/lemon/bits/traits.h
@@ -150,8 +150,8 @@
     typedef typename Map::Key Key;
     typedef typename Map::Value Value;
 
-    typedef const Value ConstReturnValue;
-    typedef const Value ReturnValue;
+    typedef Value ConstReturnValue;
+    typedef Value ReturnValue;
   };
 
   template <typename Map>
@@ -178,8 +178,8 @@
     typedef typename MatrixMap::SecondKey SecondKey;
     typedef typename MatrixMap::Value Value;
 
-    typedef const Value ConstReturnValue;
-    typedef const Value ReturnValue;
+    typedef Value ConstReturnValue;
+    typedef Value ReturnValue;
   };
 
   template <typename MatrixMap>