diff --git a/toArrayFilter.js b/toArrayFilter.js
index 722bf23..2394615 100644
--- a/toArrayFilter.js
+++ b/toArrayFilter.js
@@ -2,15 +2,22 @@ angular.module('angular-toArrayFilter', [])
 
 .filter('toArray', function () {
   return function (obj, addKey) {
-    if (!obj) return obj;
+    if ( obj === undefined ) {
+      return undefined;
+    }
+
     if ( addKey === false ) {
       return Object.keys(obj).map(function(key) {
         return obj[key];
       });
     } else {
       return Object.keys(obj).map(function (key) {
-        return Object.defineProperty(obj[key], '$key', { enumerable: false, value: key});
+        if (typeof obj[key] === "object") {
+          return Object.defineProperty(obj[key], '$key', { enumerable: false, value: key});
+        } else {
+          return obj[key];
+        }
       });
     }
   };
-});
\ No newline at end of file
+});