Unit test for a funtion in AngularJS controller which calls to a function in Onsen UI with Jasmine

I am making a unit test for a function of a controller in AngularJS code, this function will call to a function in Onsen UI code. But I fail when run it.

My code and my test code as bellow:

- In app.js file:

var module = angular.module('app', ['onsen']);
 module.controller('myctrl', ['$scope', function($scope) {
  $scope.startpop = function(index) {
    $scope.index = index;
    ons.navigator.popPage();
  };
 }]);

- in test.spec.js file:

describe("test myctrl:", function() {
  var $controller;

  beforeEach(function() {
    angular.module('onsen');
    module('app');
  });

  beforeEach(inject(function(_$controller_) {
    $controller = _$controller_;
  }));

  it('test startpop() function', function() {
    var $scope = {};
    var controller = $controller('myctrl', {$scope: $scope});
    $scope.startpop(3);

    expect($scope.index).toEqual(3);
 });

But I met error when run unit test as bellow:

TypeError: Cannot read property 'popPage' of undefined

Please help me to solve it ?