In an Angular 10 app I have a service class:

@Injectable()
export class MyService {
  private readonly bSubject1$: BehaviorSubject<SomeType[]> = new BehaviorSubject<SomeType[]>(null);
  private readonly bSubject2$: BehaviorSubject<AnotherType[]> = new BehaviorSubject<AnotherType[]>(null);
  private readonly bSubject3$: BehaviorSubject<LastType[]> = new BehaviorSubject<LastType[]>(null);
  
  private readonly subscription$: Observable<SomeType[]>;
  private readonly bs4$: BehaviorSubject<string> = new BehaviorSubject<string>('');
  private readonly bs5$ = new BehaviorSubject<boolean>(false);

  constructor(private dep1: Dep1Service) {
    this.subscription$ = this.createStructure();
  }

  private createStructure() {
    const sub$ = combineLatest([this.bSubject1$, this.bSubject2$, this.bSubject3$])
      .pipe(filter(...), map(...)); // Failed: Cannot read properties of undefined (reading 'pipe')

    return combineLatest([sub$, this.bs4$, this.bs5$])
      .pipe(map(...));
  }

}

Which I’m trying to test like this:

describe('MyService', () => {
  let service: MyService;

  beforeEach(() => {
    TestBed.configureTestingModule({
      providers: [
        { provide: dep1, useClass: Dep1Service },
        { provide: MyService, useClass: service },
      ],
    });

    service = TestBed.inject(MyService);
  });

  it('should create', () => {
    console.log('service', service);  // service is logged as `undefined`
    expect(service).toBeTruthy();
  });

});

And the test fails on the first pipe with Failed: Cannot read properties of undefined (reading 'pipe') (highlighted with a comment).

What am I missing in the test mock?