问Symfony2序列化程序有时不能工作

EN

    $em = $this->getDoctrine()->getManager('inertia');

    $entity = $em->getRepository('InertiaBundle:Accounts')->find($id);

    if (!$entity) {
        throw $this->createNotFoundException('Unable to find Accounts entity.');
    }


    if($format == 'json') {
        // return json array
        $serializer = $this->get('serializer');
        $data = $serializer->serialize($entity, 'json'); //hangs at this line.

        $response = new Response($data);
        $response->headers->set('Content-Type', 'application/json');
        return $response;
    } else {
        $deleteForm = $this->createDeleteForm($id);
        return $this->render('InertiaBundle:Accounts:show.html.twig', array(
            'entity'      => $entity,
            'delete_form' => $deleteForm->createView(),
        ));
    }