Is it possible for $_SERVER['HTTP_USER_AGENT'] to not be set?

I've just been looking through a website's error_log and one of the error's that has been logged a few times is:

[21-Jun-2011 12:24:03] PHP Notice: Undefined index: HTTP_USER_AGENT in /home/ukevents/public_html/lib/toro.php on line 130

The line this pertains to in toro.php is:

private function ipad_request() {
    return strstr($_SERVER['HTTP_USER_AGENT'], 'iPad');
}

Is it possible for $_SERVER['HTTP_USER_AGENT'] to not be set by a HTTP request?