Undefined Variable _SESSION

[eluser]MydKnight[/eluser]
I have warnings turned on, and this one identifies as merely a notice, but I wanted to be sure I wasn't doing anything wrong here. When I load the page I get the following:

A PHP Error was encountered

Severity: Notice

Message: Undefined variable: _SESSION

Filename: controllers/welcome.php

Line Number: 40

This is the code block that is causing it:

function verify()
{
if ($this->input->post('username')){
$u = $this->input->post('username');
$pw = $this->input->post('password');
$this->m_user->verifyUser($u,$pw);
if ($_SESSION['userid'] > 0){
redirect('index','refresh');
}
}
$data['main'] = 'login';
$data['title'] = "Login Page";
$data['navlist'] = array(
'home' => 'Home'
);
$this->load->vars($data);
$this->load->view('abctemplate',$data);
}

Do I have anything to be worried about?