Symfony2 @Assert \ Date（）控制器验证

I have some easy code for entity User in my project

/**
 * @Assert\NotBlank()
 * @Assert\Date()
 */
protected $dateOfBirth;

As written on Official Documentation for Date() i can send Date parameters like "YYYY-MM-DD", but i always get errors after submit in controller

$form = $this->createForm('Application\Sonata\UserBundle\Form\UserType', new User(), array(
        'csrf_protection' => false,
    ));
$form->submit($request->request->all());

Response:

"dateOfBirth": {
            "errors": [
                "This value is not valid."
            ],
            "children": {
                "year": {},
                "month": {},
                "day": {}
            }
        }