array index problem

[eluser]satterle[/eluser]
This is driving me crazy. I keep getting a message: "undefined index: on_id". I have no idea what's causing the problem. Any help greatly appreciated.

Code:

function _get_last_sibling($o_id, $on_pid)
    {
    $this->db->select('on_id');
    $this->db->where('o_id', $o_id);
    $this->db->where('on_pid', $on_pid);
    $query = $this->db->get('outline_notes');
    $row = $query->last_row('array');

    print 'on_id : '.$row['on_id'];  // print to verify, which it does

    $on_id = $row['on_id'];
    ...