Angular2 test fail when mocking a service

Trying to test a component with a service dependency that call rest API, made a mocked version of the service, with a class that extends the real service.

Now if I override the provider with the mocked one, weird error comes out of karma reporter, first 404 for 2 not existing files (that really should not exist) and then "{originalErr: {}}". I mean at least tell me what I've done wrong :)

My test suit:

 describe('App: HeaderComponent', () => {
      it('should get categories on initialization', 
        async(
          inject([TestComponentBuilder], (builder: TestComponentBuilder) => {
            let fixture: ComponentFixture<HeaderComponent>;
            expect(builder).toBeDefined();
            return builder
              .overrideProviders(HeaderComponent, [{provide: CategoryService, useClass: MockCategoryService}])
              .createAsync(HeaderComponent).then((_fixture) => {
                fixture = _fixture;
                let $el = fixture.debugElement;
                let el = $el.nativeElement;
                let component = $el.componentInstance;
                component.ngOnInit();
                fixture.detectChanges();
                expect(el.querySelector('a')).toHaveText('Electric');
              });
          })
        )
      );
    });

Output:

01 08 2016 11:58:29.684:WARN [karma]: No captured browser, open http://localhost:9876/
01 08 2016 11:58:29.713:INFO [karma]: Karma v1.1.2 server started at http://localhost:9876/
01 08 2016 11:58:29.718:INFO [launcher]: Launching browser Chrome with unlimited concurrency
01 08 2016 11:58:29.729:INFO [launcher]: Starting browser Chrome
01 08 2016 11:58:32.277:INFO [Chrome 52.0.2743 (Windows 10 0.0.0)]: Connected on socket /#Semtu7s7yqM-TW0GAAAA with id 14496897
01 08 2016 11:58:35.315:WARN [web-server]: 404: /base/dist/app/shared/services.js
01 08 2016 11:58:35.318:WARN [web-server]: 404: /base/dist/app/shared/services/mocks.js
Chrome 52.0.2743 (Windows 10 0.0.0) ERROR
  {
    "originalErr": {}
  }

---

If i remove the line the test pass (but doesn't execute the "then(...)" code block):

overrideProviders(HeaderComponent, [{provide: CategoryService, useClass: MockCategoryService}])