Error while adding validator to reactive form

I have created a simple form to which I would like to add a validator:

 form;

  constructor() { 
    this.form = new FormGroup({
    'old-password' : new FormControl('', [Validators.required, Validators.minLength(4)], PasswordValidators.checkPassword),
    'new-password' : new FormControl('', [Validators.required, Validators.minLength(4)]),
    'confirm-password' : new FormControl('', [Validators.required, Validators.minLength(4)])
  }, {
      validator: PasswordValidators.passwordsShouldMatch
    });
  }

But in validator section I get an error which says:

[ts]
Argument of type '{ validator: (control: AbstractControl) => { passwordsShouldMatch: boolean; }; }' is not assignable to parameter of type 'ValidatorFn'.
  Object literal may only specify known properties, and 'validator' does not exist in type 'ValidatorFn'.
(property) validator: (control: AbstractControl) => {
    passwordsShouldMatch: boolean;
}

When I change the construction of the form to FormBuilder it starts to working - why's that?