Setting additional user session data failing

I am attempting to set some user session data using the session class.

something like:

/**
 * Load the Session library
 */
$this->EE->load->library('session');

$this->EE->session->set_userdata('orderNumber', $orderNumber);

gives me the error:

Fatal error: Call to undefined method EE_Session::set_userdata() in /www/eh14243/public_html/control/expressionengine/libraries/Functions.php(650) : eval()’d code on line 87

however I can run this line:

$this->EE->session->set_flashdata('orderNumber', $orderNumber);

I’m assuming I can run all the CodeIgniter methods? Any ideas?

Thanks,

Jim

EDIT: This fails too.

$this->EE->session->keep_flashdata('orderNumber')