Expected validator to return Promise or Observable

Im trying to do a custom validation on Angular 5 but I’m facing the following error

Expected validator to return Promise or Observable

I just want to return an error to the form if the value doesnt match the required, heres my code:

This is the component where is my form

  constructor(fb: FormBuilder, private cadastroService:CadastroService) {
    this.signUp = fb.group({
      "name": ["", Validators.compose([Validators.required, Validators.minLength(2)])],
      "email": ["", Validators.compose([Validators.required, Validators.email])],
      "phone": ["", Validators.compose([Validators.required, Validators.minLength(5)])],
      "cpf": ["", Validators.required, ValidateCpf]

This code is in the file with the validation I want to implement:

import { AbstractControl } from '@angular/forms';

export function ValidateCpf(control: AbstractControl){
    if (control.value == 13445) {
        return {errorCpf: true}
    return null;

Can someone help me? Does that type of validation only work with observables or can I do i without being a promise or observable? thanks

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