erro iin adding offset and limit to active record

[eluser]toopay[/eluser]
$q = $this->db->select('certs.ser_cislo, certs.pocatek, certs.konec, certs.id_user, users.prijmeni as jmeno')
->join('users', 'certs.id_user=users.id')
->limit($limit, $offset)
->get('certs');