类型'Promise <validationerrors>'不能分配给'Observable <validationresult>'

Question

    static validateByService(asyncValidatorService: AbstractAsyncValidatorService): AsyncValidatorFn {
        const validityNotifier = new Subject<any>();

        return (control: AbstractControl): Observable<ValidationResult|null> => {
          if (typeof control.valueChanges === 'undefined') {
            return Observable.of({});
          };

          validityNotifier.next();
          return control.valueChanges
                        .debounceTime(AsyncValidatorDirective.DEBOUNCE_INTERVAL)
                        .takeUntil(validityNotifier)
                        .take(1)
                        .switchMap((value: any) => asyncValidatorService.validate(value));
        };
      }

      constructor() { }

      validate(control: AbstractControl): Observable<ValidationResult> {
        return AsyncValidatorDirective.validateByService(this.asyncValidatorService)(control);
  // Here getting error.
      }

错误讯息：

/src/app/validators/async-validator/async-validator.directive.ts (78,12): Type 'Promise<ValidationErrors> | Observable<ValidationErrors>' is not assignable to type 'Observable<any>'.
  Type 'Promise<ValidationErrors>' is not assignable to type 'Observable<any>'.
    Property '_isScalar' is missing in type 'Promise<ValidationErrors>'.

由于AOT编译问题，我不得不升级我的angular4。现在编译上面错误正在终端中抛出。