我正在尝试测试AngularJS服务carService，但$httpBackend似乎不起作用。

//carService
angular.module('services').factory('carService',
    function($http) {
        return {
            getTypes: function() {
                return $http.get('/api/cars/types');
            }
        };
    });

谁能解释为什么响应为空？

describe("Services", function () {

    beforeEach(module("app.services"));

    describe("Car services", function () {

        var service, $httpBackend;

        beforeEach(inject(function($injector) {
            service = $injector.get('carService');
            $httpBackend = $injector.get('$httpBackend');

            $httpBackend.when('GET', "/api/cars/types").respond(["Toyota", "Honda", "Tesla"]);
        }));

        afterEach(function() {
            $httpBackend.verifyNoOutstandingExpectation();
            $httpBackend.verifyNoOutstandingRequest();
        });

        it('getTypes - should return 3 car manufacturers', function () {
            service.getTypes().then(function(response) {
                expect(response.length).toEqual(3); //the response is null
            });
            $httpBackend.flush();
        });


    });
});