ZF2 - 找不到表单中的元素

我不明白。这是我的字段集：

<?php
namespace example'Form;
use example'Entity'User;
use example'Entity'Strategy'PasswordStrategy;
use Doctrine'Common'Persistence'ObjectManager;
use DoctrineModule'Stdlib'Hydrator'DoctrineObject as DoctrineHydrator;
use Zend'Form'Fieldset;
use Zend'InputFilter'InputFilterProviderInterface;
class UserFieldset extends Fieldset implements InputFilterProviderInterface
{
    public function __construct(ObjectManager $objectManager)
    {
        parent::__construct('user');
        $this->add(array(
            'name' => 'id',
            'type' => 'Zend'Form'Element'Hidden',
        ));
    }
    public function getInputFilterSpecification()
    {
        return array();
    }
}

这是我的表格：

<?php
namespace example'Form;
use Doctrine'Common'Persistence'ObjectManager;
use DoctrineModule'Stdlib'Hydrator'DoctrineObject as DoctrineHydrator;
use Zend'Form'Form;
use example'Form'UserFieldset;
class UserForm extends Form
{
    public function __construct(ObjectManager $objectManager)
    {
        parent::__construct('user-form');
        $userFieldset = new UserFieldset($objectManager);
        $userFieldset->setUseAsBaseFieldset(true);
        $this->add($userFieldset);
        $this->add(array(
            'name' => 'submit',
            'type'  => 'Zend'Form'Element'Submit',
            'attributes' => array(
                'value' => 'Go',
                'id' => 'submitbutton',
            ),
        ));
    }
}

我的操作：

public function userAction()
{
    $objectManager = $this->getServiceLocator()->get('Doctrine'ORM'EntityManager');
    $userForm = new UserForm($objectManager);
    return array(
        'form' => $userForm
    );
}

我的观点：

<?php
$form = $this->form;
$form->setAttribute('action', $this->url('user'));
$form->prepare();
echo $this->form()->openTag($form);
echo $this->formRow($form->get('id'));
echo $this->formRow($form->get('submit'));
echo $this->form()->closeTag();

我的错误：

在表单中找不到名称为 [id] 的元素

为什么？只有提交字段呈现良好，但是一旦我添加id字段，我就会收到错误...