(Symfony 4 Forms) 在选择字段中显式添加禁用选项

这是我的表单类型类：

  use App\Entity\User;
use App\Entity\UserSubscriptionTier;
use App\Security\UserProvider;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Form\Extension\Core\Type\ChoiceType;
use Symfony\Bridge\Doctrine\Form\Type\EntityType;
use App\Repository\UserRepository;

class FeedFilterType extends AbstractType
{
    /**
     * @var UserProvider
     */
    protected $userProvider;

    /**
     * @var UserRepository $userRepository
     */
    protected $userRepository;

    public function __construct(UserProvider $userProvider, UserRepository $userRepository)
    {
        $this->userProvider = $userProvider;
        $this->userRepository = $userRepository;
    }

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('creators', ChoiceType::class, [
                'choices'  => $this->userRepository->getCreatorsSubscribedToByUser($this->userProvider->getCurrentUser()),
                'choice_label' => 'name',
                'required' => false,
                'multiple' => true,
                'expanded' => false,
                'attr'=> array('class'=>'custom-select'),
                'choice_value' => function (User $entity = null) {
                    return $entity ? $entity->getId() : '';
                },
                'placeholder' => 'Subscriptions' /* This didn't work */ 
            ])
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array());
    }
}

我想要做的是，手动插入一个禁用选项作为我选择中的第一个选项，因此它显示为标题，例如：

我如何告诉 Symfony 表单在所有实体选项之前，我想插入一个禁用的选项作为标题？

设置'placeholder' => 'Subscriptions'不起作用。

我也尝试使用子数组，如：

               'choices' => array(
                'Subscriptions' => $this->userRepository->getCreatorsSubscribedToByUser($this->userProvider->getCurrentUser()),
                ),

但是“订阅”以斜体和粗体显示，这对我的前端人员来说不合适，因为它与 CSS 的其余部分不一致。