AngularJS HTTPBackend flush（）测试失败

Question

我正在对Karma进行角度单位测试，并有以下内容：

beforeEach(function () {
  httpBackend.when('GET', 'language/en_UK.json').respond({});
  httpBackend.expectGET('language/en_UK.json');
  httpBackend.flush();

  httpBackend.verifyNoOutstandingExpectation();
  httpBackend.verifyNoOutstandingRequest();
  httpBackend.resetExpectations();
});

此测试将失败，只有element is null行上的错误httpBackend.flush();。什么可能导致这种情况？