session destroy, not destroying session?

[eluser]x_dde_x[/eluser]
I know that I'm doing something wrong. I'm trying to kill an entire session, and effectively log a user out.

However, my logout script doesn't seem to be working

Code:

function logout()
        {        
                $this->session->sess_destroy();  
            echo $this->session->userdata('session_id');
        }

If the session is destroyed, shouldn't the session_id echo be NULL?

(for the record, this is how I'm setting the session information:

Code:

$userdata=array('session_id'=> md5($username), 'userid'=> $username);
              $this->session->set_userdata($userdata);