Validation form in Codeigniter - error in class -

i error when codeigniter validation of form:

fatal error: call member function load() on non-object in /system/libraries/form_validation.php on line 320

this full code:

$this->form_validation->set_rules('lang', 'lang', 'required|integer');         $this->form_validation->set_rules('subject', 'subject', 'required|trim|min_length[5]');         $this->form_validation->set_rules('text', 'body text', 'required|trim|min_length[5]');         $this->form_validation->set_error_delimiters('<li class="errorli">', '</li>');          if ($this->form_validation->run() == false) { } 

i error on line $this->form_validation->run().

also form_validation there in autoload in ci.