Symfony2“所选选项不存在”

I have created a FormType class called BookType. The method for generating the form is:

/**
 * @param FormBuilderInterface $builder
 * @param array $options
 */
public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder
        ->add('name', 'text', array('required'=>$this->searchForm))
        ->add('author', 'text', array('required'=>$this->searchForm))
        ->add('genre', 'text', array('required'=>$this->searchForm));

    if(!$this->searchForm) {
        $builder
            ->add('picture', 'text', array('required' => false));
    }

    $builder
        ->add('description', 'text', array('required'=>$this->searchForm))
        ->add('submit', 'submit', array('required'=>$this->searchForm))
    ;
}

However, whenever I try to access this with the following code:

$book = new Book();
$form = $this->createForm(
    new BookType(true),
    $book,
    [
        'action'=> $request->getUri()
    ]
);

I am seeing the following error message:

The option "required" does not exist. Known options are: "attr", "auto_initialize", "block_name", "disabled", "label", "translation_domain", "validation_groups".

As far as I am aware from the various tutorials I have read, this should be a completely valid parameter. Am I wrong here?

I think error is occurring here:

->add('submit', 'submit', array('required'=>$this->searchForm));

Since 'submit' field does not have 'required' option.