Getting PHP notice and warning with empty persistent collection

{
  "_id": "4c6a11810f9d503c03000000",
  "b": [
  ]
}

/** @Document(db="tests", collection="tests") */
class a
{
    /** @Id */
    protected $id;

    /** @ReferenceMany(targetDocument="b", cascade="persist") */
    protected $b;

    function getB()   {return $this->b;}
}

/** @Document(db="tests", collection="tests2") */
class b
{
    /** @Id */
    protected $id;
}

$a = $dm->loadByID('a', '4c6a11810f9d503c03000000');
echo count($a->getB()); // produces notice and warning

Notice: Undefined variable: groupedIds in library\Doctrine\ODM\MongoDB\PersistentCollection.php on line 122

Warning: Invalid argument supplied for foreach() in library\Doctrine\ODM\MongoDB\PersistentCollection.php on line 122