Index: wp-includes/vars.php
===================================================================
--- wp-includes/vars.php	(revision 11902)
+++ wp-includes/vars.php	(working copy)
@@ -37,6 +37,9 @@
 // Simple browser detection
 $is_lynx = $is_gecko = $is_winIE = $is_macIE = $is_opera = $is_NS4 = $is_safari = $is_chrome = $is_iphone = false;
 
+if ( !isset($_SERVER['HTTP_USER_AGENT']) )
+	$_SERVER['HTTP_USER_AGENT'] = '';
+
 if (strpos($_SERVER['HTTP_USER_AGENT'], 'Lynx') !== false) {
 	$is_lynx = true;
 } elseif ( strpos(strtolower($_SERVER['HTTP_USER_AGENT']), 'chrome') !== false ) {