Session Library another Error

[eluser]Zorancho[/eluser]
Hi all.
I found this website where some of the guys from CodeIgniter is having a tutorial about protection using the session library.
His code was something like this:

Code:

class User extends Controller
{
   function User
   {
      parent::Controller();
      if($this->session->userdata('hash_string'))
      {
      $this->session->userdata(array('last_hash_string' =>   $this->session->userdata('hash_string')));
      }
    $this->load->helper('string');
    $this->session->userdata(array('hash_string' => random_string('alnum', 6)));
}

And then he puts the hash_string from the session into a hidden input that he checks with the form_validation library if it is the same with the last_hash_string...
But when i try it i get this error:
Severity: Warning

Message: Illegal offset type in isset or empty

Filename: libraries/Session.php

Line Number: 422

And that line is:

Code:

function userdata($item)
{
    return ( ! isset($this->userdata[$item])) ? FALSE : $this->userdata[$item];
}

If hash_string it's not isset then return false, otherwise it should return the value of it. But i don't get it why it doesn't see the given argument as valid.
Any ideas?
Thanks in advance