I tested this with a blank user agent and can't figure out why the code is logging errors. Any tips?

[01-May-2011 11:42:44] PHP Warning: strpos() [<a href='function.strpos'>function.strpos</a>]: Empty delimiter in /home/site/public_html/file.php on line 41
Line 41:
foreach ($arr as $bot) {
if (strpos($bot, $agent)) {
$bot_here = true;
}
}