extending CI_form_validation don't work

[eluser]Unknown[/eluser]
So i extended my form_validation library.

Code:

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class Lt_form_validation extends CI_Form_validation {
    
    public function __construct() {
        parent::__construct();
    }

    public function clear_field_data() {

        $this->_field_data = array();
        return $this;
    }
    
}

but when i try to reach it, i get error:

Code:

Fatal error: Call to undefined method CI_Form_validation::clear_field_data()

I trying to call like that

Code:

$this->form_validation->clear_field_data();

What i am doing wrong?