how to fix this error?

[eluser]chmod[/eluser]
I am sure the problem in these codes:

Code:

$this->form_validation->set_rules('sig_key', 'lang:sig_key', 'trim|xss_clean|required|callback_checkSigKey');

my callback function:

Code:

function checkSigKey(){
         if ($this->session->userdata('sig_key') !== $this->input->post('sig_key')){
             $this->form_validation->set_message('checkSigKey', $this->lang->line('sig_key_invalid'));
             return false;
         }else{
             return true;
         }
     }

I can not find any errors.

some one help me?