Selecting records where column is NULL

Hi,

I'm trying to use the ORM to select records from the database where the column value is NULL. Does anyone know how to achieve that? I've tried:

Groups::find(array( 'conditions' => 'parent_id=NULL' ));

Groups::find(array( 'conditions' => 'parent_id=?0', 'bind' => array(null) ));

Groups::find(array( 'conditions' => 'parent_id=?0', 'bind' => array(new \Phalcon\Db\RawValue('NULL')) ));

Thank You!