diff --git a/src/ng/cacheFactory.js b/src/ng/cacheFactory.js
index a6fcf233..c5914649 100644
--- a/src/ng/cacheFactory.js
+++ b/src/ng/cacheFactory.js
@@ -67,8 +67,10 @@
            $scope.keys = [];
            $scope.cache = $cacheFactory('cacheId');
            $scope.put = function(key, value) {
-             $scope.cache.put(key, value);
-             $scope.keys.push(key);
+             if ($scope.cache.get(key) === undefined) {
+               $scope.keys.push(key);
+             }
+             $scope.cache.put(key, value === undefined ? null : value);
            };
          }]);
      </file>