Async function leads to time out in Angular Karma testing

Karma is making me confused again.. This is how the test looks like, I call httpMock.verify() in afterEach(..). The error message is Error: Timeout - Async function did not complete within 5000ms.

it('search user service should send GET request and return data', (done) => {
    let filter = '?filter=hans'
    inject([HttpTestingController, SearchUserService],
        (httpMock: HttpTestingController, service: SearchUserService) => {
            service.findUser(filter).subscribe(data => {
                expect(data).toBeTruthy();
                done();
            });

            const req = httpMock.expectOne(`http://localhost:8080/api/rest/frontend/user${filter}`)
            expect(req.request.method).toEqual('GET')

            req.flush({
                data:
                 {
                     ..
                 }
            });
    })
  });

The findUser() function returns an Observable<any>. What is wrong here?