Symfony2 doctrine selection criteria

I have an Entity called "Machine" with an association with Entity "Event"

/**
 * @var ArrayCollection
 *
 * @ORM\OneToMany(targetEntity="Event", mappedBy="machine", fetch="EXTRA_LAZY", cascade={"persist", "remove"})
 */
private $events;

      

        
        
        
      

    

In my twig template I am iterating over the "Machine" items

{% for machine in machines %}
    <tr>
        <td>{{ machine.name }}</td>
        <td>{{ machine.lastEvent.value }}</td>
    </tr>
{% endfor %}

      

        
        
        
      

    

I want to get the latest event for each car

/**
 * Get machine current event value
 *
 * @return Event
 */
public function getLastEvent()
{
    $expression = Criteria::expr()
        ->eq('machine', $this->id);

    $criteria = Criteria::create()
        ->setMaxResults(1)
        ->where($expression)
        ->orderBy(['id' => 'DESC']);

    return $event = $this
        ->getEvents()
        ->matching($criteria)
        ->first();
}

      

        
        
        
      

    

When running this error, I am getting the following error:

"Cannot match on ApiBundle \ Entity \ Event :: machine with a non-object value. Object matching by id is incompatible with matching against an in-memory collection that compares objects by reference.

What am I doing wrong?