TypeError: Caot read property '$$hashKey' of undefined

I have an object retu by http post in angular

var altest = {

//here I have an empty object prepared to be extend

  Detail: {}


  getVacanshitDetail: function(passdata) {
    var self = this;      
    $http.post(Constants.API.baseUrl).success( function(response) {
        console.log(response);
        angular.extend(self.Detail,response);
    });
  },

However, in the extend line, in the console log in says: TypeError: Caot read property '$$hashKey' of undefined

//object response from http request 

Object {
    title:  'Frog',
    url:    '/img/picture.jpg',
    width:  300,
    height: 200
  };

Please help what I need to do to achieve what I want and avoid the error.

Many thanks in advance.