Error with base_url()

Hi everyone, 

I'm actually trying to put online my website.
In local, everything goes fine, but when i send my files on the server (one.com), this message appear when i go to the site: 

"An uncaught Exception was encountered

Type: Error
Message: Call to undefined method CI_Controller::base_url()
Filename: /customers/f/1/b/onsen.be/httpd.www/application/views/errors/html/error_404.php
Line Number: 9
Backtrace:
File: /customers/f/1/b/onsen.be/httpd.www/index.php
Line: 315
Function: require_once"

And there is my error_404.php file:

"<?php



$CI =& get_instance();

if( ! isset($CI))

{

    $CI = new CI_Controller();

}

$CI->load->helper('url');

echo $CI->base_url();
?>
"
I'm working with Codeigniter 3.0.6

Can someone help me with this ? 
It's relly important for me to fix this as soon as possible.

Thanks for the help !