$this->session->set_userdata seems broken?

[eluser]cpass78[/eluser]
Hello everyone,
I seem to be having an issue after upgrading to CI 2.1.0 and the set_userdata function. Previously running on CI 2.0.3(?) everything was working fine.

So I have this small block of code:

Code:

$details = $this->getClientDetails($validlogin['userid']);
                $sessiondata = array('username' => $details['firstname'], 'passwordhash'=>$validlogin['passwordhash'], 'logged_in' => '1', 'userid' => $details['userid'], 'status' => $details['status']);
                $this->session->set_userdata($sessiondata);

Which functions properly up to the point where $this->session->set_userdata($sessiondata); gets called. below is what happens in the DB table, as you can see "user_data" is empty and seems to be skipping it completely?.

Code:

a:6:{s:9:"user_data";s:0:"";s:8:"username";s:5:"chris";s:12:"passwordhash";s:32:"bbsdbsdbtrtgh45hhhwedf";s:9:"logged_in";s:1:"1";s:6:"userid";s:1:"2";s:6:"status";s:6:"Active";}

Any ideas?

Thank you