Form Type Extension Magic

https://knpuniversity.com/s...

I think it will be good to remind about Symfony3.3-like adding service:

AppBundle\Form\TypeExtension\HelpFormExtension:
tags:
- { name: form.type_extension, extended_type: Symfony\Component\Form\Extension\Core\Type\FormType }

Hi, I get this error at the end:
Type error: Argument 1 passed to Symfony\Component\OptionsResolver\OptionsResolver::setDefaults() must be of the type array, string given, called in C:\Users\John\Documents\Siti\aqua_note\src\AppBundle\Form\TypeExtension\HelpFormExtension.php on line 22
I think It shouldn't be an array because it's a value and its option, what am I doing wrong? Below my code:
namespace AppBundle\Form\TypeExtension;

use Symfony\Component\Form\AbstractTypeExtension;
use Symfony\Component\Form\Extension\Core\Type\FormType;
use Symfony\Component\Form\FormInterface;
use Symfony\Component\Form\FormView;
use Symfony\Component\OptionsResolver\OptionsResolver;

class HelpFormExtension extends AbstractTypeExtension
{
public function buildView(FormView $view, FormInterface $form, array $options)
{
if ($options['help']) {
$view->vars['help'] = $options['help'];
}
}
public function configureOptions(OptionsResolver $resolver)
{
$resolver->setDefaults('help', null);
}
public function getExtendedType()
{
return FormType::class;
}
}

Thanks in advance