"IS NOT NULL" expression with Symfony query builder

I searched for a easy way to get values that contain a not null in the where clause. That's what I came up with:

$repo = $this->getDoctrine()->getRepository('LiplexBundle:User');
$query = $repo
    ->where('u.lastOnline > :lastOnline')
    ->andWhere('u.posts IS NOT NULL')
    ->setParameter('lastOnline', new \DateTime('2 days ago'))

There is also a isNotNull() expression, but the only thing it does is returning a concatenated string with IS NOT NULL:

 * Creates an IS NOT NULL expression with the given arguments.
 * @param string $x Field in string format to be restricted by IS NOT NULL.
 * @return string
public function isNotNull($x)
    return $x . ' IS NOT NULL';

So there is no good reason for using anything other then just plain DQL here.