Assert entity is selected if checkbox is checked with Expression Constraint - Symfony2 Form Validation

DormerRequestType.php

<?php

namespace AppBundle\Form;

use Symfony\Component\Form\FormBuilderInterface;

class DormerRequestType extends BaseRequestType
{
    public function buildForm(FormBuilderInterface $builder, array $options) {
        switch ($options['flow_step']) {
            case 1:
                parent::buildForm($builder, $options);
                break;
            case 2:
                $builder
                    ->add('dormerRoofOverhang', 'choice', array(
                        'choices' => array(
                            '0' => 'Nein',
                            '1' => 'Ja'
                        ),
                        'expanded' => true,
                        'required' => true,
                        'translation_domain' => 'dormer'
                    ))
                    ->add('dormerRoofOverhangWidth', 'entity', array(
                        'class' => 'Acme\AppBundle\Entity\RoofOverhangWidth',
                        'translation_domain' => 'dormer'
                    ))
                ;
                break;
        }
    }

    public function getName() {
        return 'dormerRequest';
    }
}

PriceQuoteRequest.php

<?php

namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * @Assert\Expression("this.getDormerRoofOverhang() == 1 & this.getDormerRoofOverhangWidth() != null", groups={"flow_dormerRequest_step2"})
 */
class PriceQuoteRequest
{
    /**
     * @var $dormerRoofOverhang
     *
     * @ORM\Column(name="roof_overhang", type="integer")
     *
     * @Assert\NotBlank(groups={"flow_dormerRequest_step2"})
     * @Assert\Type(type="integer", groups={"flow_dormerRequest_step2"})
     */
    public $dormerRoofOverhang;

    /**
     * @var $dormerRoofOverhangWidth
     *
     * @ORM\OneToOne(targetEntity="AppBundle\Entity\RoofOverhangWidth")
     * @ORM\JoinColumn(name="roofOverhangWidth_id", referencedColumnName="id")
     *
     * @Assert\Type(type="AppBundle\Entity\RoofOverhangWidth", groups={"flow_dormerRequest_step2"})
     * Assert\Expression("this.getDormerRoofOverhang() == 1 & this.getDormerRoofOverhangWidth() == null", groups={"flow_dormerRequest_step2"})
     */
    public $dormerRoofOverhangWidth;
}

Goal:
If the checkbox for dormerRoofOverhang has been checked for value 1 then an entity RoofOverhangWidth has to be selected from dormerRoofOverhangWidth.

missed a @ in line 30?