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?