Symfony Entity Listener with Swift Mailer and Twig Environment has empty Entity

Question

I have read the Symfony Documentation on Entity Listeners and Doctrine Documentation on Entity Listeners in addition to this answer and this blog post by Eric Geloen but I can not get my listener to do what I want. In this entity I have a field called status, and when this field changes, an email is going to be sent notifying the receiver of the change.

Entity

I have an entity shipmentLine with the following annotations:

<?php
namespace Acme\Bundle\ShipmentBundle\Entity;
use Doctrine\ORM\Mapping as ORM;
use JMS\Serializer\Annotation\ExclusionPolicy;
use JMS\Serializer\Annotation\Expose;

/**
 * @ORM\Entity
 * @ORM\HasLifecycleCallbacks
 * @ORM\EntityListeners({"\Acme\Bundle\ShipmentBundle\Listener\ShipmentLineListener"})
 * @ExclusionPolicy("all")
 */
class ShipmentLine
{
    . . .
}

Listener

The ShipmentLineListener has implemented the __construct and prePersist functions:

<?php
namespace Acme\Bundle\ShipmentBundle\Listener;
use Doctrine\ORM\Event\LifecycleEventArgs;
use Acme\Bundle\ShipmentBundle\Entity\ShipmentLine;

class ShipmentLineListener
{
    private $mailer;
    private $twig;

    public function __construct(\Swift_Mailer $mailer, \Twig_Environment $twig)
    {
        $this->mailer = $mailer;
        $this->twig = $twig;
    }

    public function prePersist(ShipmentLine $shipmentLine, LifecycleEventArgs $event)
    {
        /** @var \Swift_Mime_Message $message */
        $message = \Swift_Message::newInstance()
            ->setSubject('Entity Listener test')
            ->setFrom(array('[email protected]' => 'Acme'))
            ->setTo(array('[email protected]' => 'Tommy (Acme)'))
            ->setBody(json_encode($event), 'text/html');

        $this->mailer->send($message);
    }
}

services.yml

Lastly I have registered my listener in services.yml

entity.entity_listener.shipmentline_update:
    class: Acme\Bundle\ShipmentBundle\Listener\ShipmentLineListener
    arguments: [ "@mailer", "@twig" ]
    tags:
        - { name: doctrine.orm.entity_listener, event: prePersist }

The listener fires, and the mail is sent. But both the $shipmentLine and $event argument is empty. Can someone see if I do anything wrong? I have a hard time seeing any errors from what I have read.

Thank you for your time.