Angular : unit test fail when inject a service

I started unit testing to existing Angular/ionic App i have install karma and jasmine. I wrote the first unit test for a service but it fail and got this error:

Error: [$injector:unpr] Unknown provider: followServiceProvider <- followService

I wonder if my injecting way is correct! To make sure my setup is fine i started new ionic app installed karma and jasmine tried simple test and it worked

Here is the code of the service followService:

'use strict';

 angular.module('myApp.services')
    .factory('followService', function($http, $q, API_URL) {

        var url = API_URL + 'api/users/followers' ;

        function getFollowers() {
            var deferred = $q.defer();
            $http.put(url)
                .success(function(data, status, headers, config) {
                    deferred.resolve(data);
                })
                .error(function(data, status, headers, config) {
                    deferred.reject(data);

                });
            return deferred.promise;
        }

    });

And this the unit test for it :

'use strict';

describe('Service: followService Test Follow APIs', function () {
    // load the service's module
    beforeEach(module('myApp.services'));

    // instantiate service
    var followService;
    var $httpBackend;

    beforeEach(inject(function (_$httpBackend_, _followService_) {
        followService = _followService_;
        $httpBackend = _$httpBackend_;
    }));

      it('should GET Followers from the server', function () {
        $httpBackend.expectGET('api/users/followers').respond(200, {"id": "55d1c1acddb6dabe468bbba1", "displayName": "Brand1", "pictrue": "path/to/picture"});

        var followers = followService.getFollowers();

        $httpBackend.flush();

        expect(followers).not.toBe(null);
    });
});