Codeigniter: problem extending Database Driver

Im using Codeigniter 2.x and been trying to extend the database class and followed the wiki but im getting this error:

Fatal error: Uncaught Error: Call to undefined method MY_DB_mysqli_driver::where() in /system/libraries/Session.php on line 218

Error: Call to undefined method MY_DB_mysqli_driver::where() in /system/libraries/Session.php on line 218

On line 218 of /system/libraries/Session.php:

$this->CI->db->where('session_id', $session['session_id']); 

My extended class is just the example from the wiki:

<?php
class MY_DB_mysqli_driver extends CI_DB_mysqli_driver
{
    public function __construct($params)
    {
        parent::__construct($params);
        log_message('debug', 'Extended DB driver class instantiated!');
    }

    public function get_first($table)
    {
        return $this->limit(1)->get($table);
    }
} 

from Codeigniter: problem extending Database Driver