Fatal error: Non-abstract method News_model::__construct() must contain body [CodeIgniter]

i am trying the tutorial in CodeIgniter User Guide Version 2.2.0. i just followed the code pasted but i got this error message. what is really wrong here?

Fatal error: Non-abstract method News_model::__construct() must contain 
body in /var/www/leomel/application/models/news_model.php on line 3

the source code:

<?php
class News extends CI_Controller {
   public function __construct()
   {
     parent::__construct();
     $this->load->model('news_model');
   }

   public function index()
   {
     $data['news'] = $this->news_model->get_news();
     $data['title'] = 'News archive';

     $this->load->view('templates/header', $data);
     $this->load->view('news/index', $data);
     $this->load->view('templates/footer');
   }

   public function view($slug)
   {
     $data['news'] = $this->news_model->get_news($slug);
     if (empty($data['news_item']))
     {
        show_404();
     }

     $data['title'] = $data['news_item']['title'];

     $this->load->view('templates/header', $data);
     $this->load->view('news/view', $data);
     $this->load->view('templates/footer');
   }
}