diff --git a/index.js b/index.js
index 7f657b9..9ca1593 100644
--- a/index.js
+++ b/index.js
@@ -43,7 +43,9 @@ var keyMirror = function(obj) {
   }
   for (key in obj) {
     if (obj.hasOwnProperty(key)) {
-      ret[key] = key;
+      if(ret[key]==null){
+        ret[key] = key;
+      }
     }
   }
   return ret;