Symfony2 EntityManager#persist() expects parameter

Question

I am new to Symfony2 and can't to figure out why I'm getting this error. Perhaps there is something wrong with my Entities? So I need some help with it. An error:

     EntityManager#persist() expects parameter 1 to be an entity object, array given.
500 Internal Server Error - ORMInvalidArgumentException 

I have the following code:

User.php:

    <?php

namespace Acme\MainBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * User
 */
class User
{
    /**
     * @var integer
     */
    private $id;

    /**
     * @var string
     */
    private $username;

    /**
     * @var string
     */
    private $password;


    /**
     * Get id
     *
     * @return integer 
     */
    public function getId()
    {
        return $this->id;
    }

    /**
     * Set username
     *
     * @param string $username
     * @return User
     */
    public function setUsername($username)
    {
        $this->username = $username;

        return $this;
    }

    /**
     * Get username
     *
     * @return string 
     */
    public function getUsername()
    {
        return $this->username;
    }

    /**
     * Set password
     *
     * @param string $password
     * @return User
     */
    public function setPassword($password)
    {
        $this->password = $password;

        return $this;
    }

    /**
     * Get password
     *
     * @return string 
     */
    public function getPassword()
    {
        return $this->password;
    }
}

UserRegType.php

    <?php
namespace Acme\MainBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Validator\Constraints\Length;

class UserRegType extends AbstractType 
{
    public function buildForm(FormBuilderInterface $builder, array $options) {
        $builder->add('username', 'text', array(
            'constraints'=>new Length(array('min' => 3))
        ));
        $builder->add('password', 'repeated', array(
            'first_name' => 'password',
            'second_name' => 'confirm',
            'type' => 'password',
            'constraints' => new Length(array('min' => 5, 'max' => 8))
        ));
    }

    public function getDefaultOptions(array $options)
    {
        return array('data_class' => 'Acme\MainBundle\Entity\User');
    }

    public function getName() {
        return 'user';
    }
}
?>

UserController.php:

    <?php

namespace Acme\MainBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;

use Acme\MainBundle\Form\Type\UserRegType;

class UserController extends Controller
{
    public function registerAction()
    {    
        $form = $this->createForm(new UserRegType());
        return $this->render(
            'AcmeMainBundle:User:register.html.twig',
            array('form' => $form->createView())
        );    
    }

    public function createUserAction()
    {
        $em = $this->getDoctrine()->getEntityManager();

        $form = $this->createForm(new UserRegType());

        $form->bind($this->getRequest());
        $username = $form["username"]->getData();

        if ($form->isValid()) {
            $reg = $form->getData();
            $em->persist($reg);
            $em->flush();
            $session = $this->get('session'); 
            $session->set('username', $username);    

            return $this->redirect($this->generateUrl('home'));
        }
    }

    public function loginAction()
    {
    }

    public function logoutAction()
    {
    }


}