1064 error with CodeIgniter in ci_sessions table

// Run the update query
    $this->CI->db->where('session_id', $this->userdata['session_id']);
    $this->CI->db->update($this->sess_table_name, array('last_activity' => $this->userdata['last_activity'], 'user_data' => $custom_userdata));

I have a problem using sessions in CodeIgniter, I downloaded de las CI version and I have to modify many things to make it works, but, I have this f.. problem, when I'm trying to login in my web, I have this problem, with 1064 error MySQL

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'SET `last_activity` = 1430854831, `user_data` = 'a:8:{s:9:\"user_data\";s:0:\"\"' at line 1

UPDATE SET `last_activity` = 1430854831, `user_data` = 'a:8:{s:9:\"user_data\";s:0:\"\";s:10:\"id_cliente\";s:1:\"2\";s:4:\"name\";s:14:\"Martín Sosa\";s:8:\"lastName\";s:6:\"Plaz\";s:9:\"lastName2\";s:4:\"Loah\";s:15:\"numberExpedient\";s:5:\"00002\";s:5:\"email\";s:27:\"mart94@hotmail.com\";s:3:\"rol\";s:1:\"1\";}' WHERE `session_id` = '6b4cceb3679bdc63a4534c6be0efd54f'

Filename: libraries/Session.php

Line Number: 289