Form Validation Error

[eluser]xtremer360[/eluser]
I'm working on my register form and I have it submitting via jQuery with having some validation checks client side and then of course on the server side but I get an error when I hit submit.

When I submit the form I get this message:

<b>Fatal error</b>: Call to private method Register::username_exists() from context 'CI_Form_validation' in <b>/home/xtremer/public_html/system/libraries/Form_validation.php</b> on line <b>593</b><br />

Code:

$this->form_validation->set_rules('username', 'Username', 'trim|required|xss_clean|min_length[6]|max_length[12]|alpha_numeric|strtolower|callback_username_exists');

Code:

private function username_exists($str)
{
    if ($this->users_model->is_username_available($this->input->post('username')))
    {
$this->form_validation->set_message('username', 'Testing');
return FALSE;
    }
    else
    {
        return TRUE;
    }
}