Cannot call the same static method from identical method from the same class

I have this error controller in my Codeigniter 2.1.0 application:

<?php

class Error extends CI_Controller
{
    public function __construct()
    {
        parent::__construct();
    }

    public function index()
    {
        set_status_header(404);
        $data->menuItems = Main::_menu();
        $data->title = "404 error !";
        $data->pageview = 'templates/404';
        $this->load->view('templates/main', $data);
    }

    public function facebook()
    {

        set_status_header(404);            
        $data->menuItems = Main::_menu();
        $data->title = "Facebook error !";
        $data->pageview = "templates/facebook_error";
        $this->load->view('templates/main', $data);
    }
}
?>

The Maincontroller _menu:

<?php
class Main extends CI_Controller
{
    // ...  a lot of methods here ...
    public static function _menu()
    {
            static $menuItems = array( //just a simple array
                                     );
    }
}
?>

facebook() method is totally the same as the index(), however index works fine, facebook() throw this message:

Fatal error: Class 'Main' not found in /var/www/MYApplicationName/application/controllers/error.php on line 22

How the earth is that possible ? How can I reach Main::_menu() from facebook() method ?