php - Mongodb doctrine "zero-length keys are not allowed, did you use $ with double quotes"

Question

programmers, I uses symfony doctrine and mongodb. Today I tried to create following query:

$q = $this->createQueryBuilder("user");
$keywords = explode("_", $keyword);
foreach ($keywords as $keyword) {
    $r = new \MongoRegex('/'.$keyword.'/i');
    $q->addAnd(
    $q->addOr($q->expr()->field('firstName')->equals($r))
        ->addOr($q->expr()->field('lastName')->equals($r))
    );
}
return $q->getQuery()->execute();

but after execute() i got following error "zero-length keys are not allowed, did you use $ with double quotes". query must realize user search by keywords. can you help me fix that, please?