AngularJS $httpBackend asynchronous response

Question

I'm trying to mock the back-end for an AngularJS(1.3.8)-app with ngMockE2E as replacement until the back-end code has been written.

I'm using already existing services that also query other data, however they return a promise. I am aware that ngMockE2E is supposed to be synchronous, however I wanted to see if there's a way to do it asynchronously first.

Looking around the web I found this and put the mocking-related code into its own seperate module to see if this approach works.

 $httpBackend.whenAsync('projects/').respond(function (promise, headers, status) {
    var deferred = $q.defer();
    _getProjectIndex().then(function (result) {
            deferred.resolve(result);
        },
        function (statusCode) {
            console.log(statusCode);
            deferred.reject(statusCode);
        });
    return deferred.promise;
});

When I try to run $httpBackend.whenAsync() the request just seems to 404. Checking the same request with $httpBackend.whenGET() I receive the promise containing the data I requested.

What am I doing wrong?