codeigniter中的注销错误

I am trying to design a login/logout page in code igniter framework.My problem is that when I logout of the web-page I am getting redirected to a login page. When I go back I am getting a page which says:

Document Expired

This document is no longer available.

My Login controller is

public function __construct() {
        parent::__construct();

        //date_default_timezone_set("Asia/Kolkata");
        $this->load->helper(array('form', 'url'));
        $this->load->database();
        $this->load->model('User_model');
        $this->load->library(array('session', 'form_validation', 'email'));
        $this->load->file('smtp/Send_Mail.php');

        $this->output->set_header('Last-Modified: ' . gmdate("D, d M Y H:i:s") . ' GMT');
        $this->output->set_header('Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0');
        $this->output->set_header('Pragma: no-cache');
        $this->output->set_header("Expires: Mon, 26 Jul 1997 05:00:00 GMT");    

    }

function Logout() {

        $user_data = $this->session->all_userdata();
        foreach ($user_data as $key => $value) {
            if ($key != 'session_id' && $key != 'ip_address' && $key != 'user_agent' && $key != 'last_activity') {
                $this->session->unset_userdata($key);
            }
        }
        $this->session->sess_destroy();
        redirect('Login');
    }

<?php
if($this->session->is_user_login==TRUE)
{
   redirect('User/Emailverification');
}
?>

I am not getting how to resolve this document expired error.I have dono lots of searching but i am not getting the solution of this problem.Please help me thanks in Advance.