Doctrine createQuery Uncaught异常

I trying use SELECT by using doctrine orm. Database: postgresql

$dql = 'SELECT name_serial FROM serial';
$query = $entityManager->createQuery($dql);
$query->setMaxResults(30);
$result = $query->getResult();
echo json_encode($result);

But have exception:

Uncaught Doctrine\ORM\Query\QueryException: SELECT name_serial FROM serial in /home/{username}/Serials/vendor/doctrine/orm/lib/Doctrine/ORM/Query/QueryException.php:41

Somebody can help?