Index: comment.php
===================================================================
--- comment.php	(revision 23180)
+++ comment.php	(working copy)
@@ -1388,7 +1388,7 @@
 	$commentdata['comment_parent'] = ( 'approved' == $parent_status || 'unapproved' == $parent_status ) ? $commentdata['comment_parent'] : 0;
 
 	$commentdata['comment_author_IP'] = preg_replace( '/[^0-9a-fA-F:., ]/', '',$_SERVER['REMOTE_ADDR'] );
-	$commentdata['comment_agent']     = substr($_SERVER['HTTP_USER_AGENT'], 0, 254);
+	$commentdata['comment_agent']     = ( isset($_SERVER['HTTP_USER_AGENT']) ? substr($_SERVER['HTTP_USER_AGENT'], 0, 254) : '' );
 
 	$commentdata['comment_date']     = current_time('mysql');
 	$commentdata['comment_date_gmt'] = current_time('mysql', 1);