$_SERVER['HTTP_REFERER'] not work!!

[eluser]mashary[/eluser]
i create my own 404 page
then i add this script $_SERVER['HTTP_REFERER']
but it say error
why?

Code:

A PHP Error was encountered

Severity: Notice

Message: Undefined index: HTTP_REFERER

Filename: views/notfound.php

Line Number: 22

[eluser]Aken[/eluser]
Some servers may not provide http referer information. More info on php.net

[eluser]mashary[/eluser]
thanks