php - Fatal error: Call to undefined function validation_error() -

i try use codeigniter, , when use 'validation_error();' it's show 'fatal error: call undefined function validation_error()' on browser.
view code,

<?php  echo form_open('main/login_validation');  echo validation_error();  echo "<p>email: "; echo form_input('email'); echo "</p>";  echo "<p>password: "; echo form_password('password'); echo "</p>";  echo "<p>"; echo form_submit('login_submit', 'login'); echo "</p>";  echo form_close();  ?> 

this controller,

public function login_validation(){      $this->load->library('form_validation');     $this->form_validation->set_rules('email', 'email', 'required|trim|xss_clean');     $this->form_validation->set_rules('password', 'password', 'required|md5|trim');      if($this->form_validation->run()){         redirect('main/members');     } else{         $this->load->view('login');     } } 

and 1 of autoload,

$autoload['helper'] = array('form', 'url'); 

please :(