Angular2 form disable submit

I put a recaptcha in a signup form with angular2. I have a controller to match the recaptcha with a validator required.

this.captchaControl = fb.control(false, Validators.required); 
this.userForm = fb.group({
    ...
    captchaControl: this.captchaControl
});

And the value is correctly set to true when the user answer it.

checkReCaptcha(response){
    console.log(this.userForm.valid);
    this.captcha = response;
    this.captchaControl.updateValue(true);
    console.log(this.userForm.valid);
}

The first console log show false, the second print true.

In the submit button, I put the disable function:

<button type="submit" [disabled]="!userForm.valid" class="btn btn-d">Register</button>

But the button stay disable.

How can I trigger the button to check again the validity of userForm ?