Notice trying to match a Criteria referencing a managed entity not yet persisted

Undefined index: (...) in Doctrine/ORM/UnitOfWork.php line 2735

#0 doctrine/orm/lib/Doctrine/ORM/UnitOfWork.php(2735): Symfony\Component\HttpKernel\Debug\ErrorHandler->handle(8, 'Undefined index...', '/Users/frosas/S...', 2735, Array) 
#1 doctrine/orm/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php(1696): Doctrine\ORM\UnitOfWork->getEntityIdentifier(Object(MyEntity)) 
#2 doctrine/orm/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php(1683): Doctrine\ORM\Persisters\BasicEntityPersister->getIndividualValue(Object(MyEntity)) 
#3 doctrine/orm/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php(818): Doctrine\ORM\Persisters\BasicEntityPersister->getValue(Object(MyEntity)) 
#4 doctrine/orm/lib/Doctrine/ORM/Persisters/BasicEntityPersister.php(786): Doctrine\ORM\Persisters\BasicEntityPersister->expandCriteriaParameters(Object(Doctrine\Common\Collections\Criteria)) 
#5 doctrine/orm/lib/Doctrine/ORM/PersistentCollection.php(835): Doctrine\ORM\Persisters\BasicEntityPersister->loadCriteria(Object(Doctrine\Common\Collections\Criteria)) 
#6 (...): Doctrine\ORM\PersistentCollection->matching(Object(Doctrine\Common\Collections\Criteria)) 
#7 (...)

if ($myEntity->getId() === null) return null;
$criteria = Criteria::create()->where(Criteria::expr()->eq('field', $myEntity));
return $this->collection->matching($criteria)->first();