Symfony 2 form optional image

I have an issue with my Symfony 2 form and my image upload.

I created a form :

$form = $formFactory->create(new PlaceType($this->_em), $place, array(
                'method' => 'POST',
                'status' => $place->getStatus()
        ));

In PlaceType.php :

public function buildForm(FormBuilderInterface $builder, array $options) {
            $builder->add('images', 'collection', array(
                    'type' => new ImageType($this->em),
                    'allow_add' => true,
                    'data' => array(new Image())));
    }

In ImageType.php :

public function buildForm(FormBuilderInterface $builder, array $options) {
    $builder->add('file', 'file', array('required' => false));
}

It's working very well, but when I don't fill the fiel field, I have an exception threw because file infos are missing.

I would like to make this field optional. I tried a lot of solutions, but none is working.

Thanks.