Form validation: Callback not changing the input value

[eluser]coolgeek[/eluser]
I've got a problem trying to extend the form validation library

Code:

<?php

class Add_member_award extends Controller {

    function Add_member_award()
    {
        parent::Controller();
    }

    function index() {


        $data['mview'] = "add_member_award";

        $this->load->library('form_validation');

        $this->form_validation->set_rules('award', 'Award', 'trim|required|xss_clean|callback_award_check');

        if ($this->input->post('submit')){
            if ($this->form_validation->run() == TRUE) {
                $award = $this->input->post('award');

                $this->load->model('Mmember_award');
                $this->Mmember_award->createMemberAward($uid, $award);
                redirect("member/$uid");
            }
        }
        $this->load->vars($data);
        $this->load->view('template', $data);
    }
}

class MY_Form_validation extends CI_Form_validation {

    function My_Form_validation()
    {
        parent::CI_Form_validation();
    }


    function award_check($str)
    {
        if ($str == 'test award')
        {
            $this->form_validation->set_message('award_check', 'The %s field can not be the word "test award"');
            return FALSE;
        }
        else
        {
            return TRUE;
        }
    }
}

If I put function award_check in the controller, form validation triggers the check as expected. But it's not triggering it in MY_Form_validation.

What am I doing wrong here?

Thanks