Wrong Async Validator Return Type

Async validators must return a promise or an observable, and emit/resolve them whether the validation fails or succeeds. In particular, they must implement the AsyncValidatorFn API

export function isTenAsync(control: AbstractControl): Observable<ValidationErrors | null> {
  const v: number = control.value;
  if (v !== 10) {
    // Emit an object with a validation error.
    return of({'notTen': true, 'requiredValue': 10});
  }
  // Emit null, to indicate no error occurred.
  return of(null);
}

Debugging the error

Did you mistakenly use a synchronous validator instead of an async validator?