=== wp-blog-header.php
==================================================================
--- wp-blog-header.php  (revision 2652)
+++ wp-blog-header.php  (local)
@@ -135,7 +135,7 @@
 
 	if ( ($client_last_modified && $client_etag) ?
 		((strtotime($client_last_modified) >= strtotime($wp_last_modified)) && ($client_etag == $wp_etag)) :
-		((strtotime($client_last_modified) >= strtotime($wp_last_modified)) || ($client_etag == $wp_etag)) ) {
+		(($client_last_modified && strtotime($client_last_modified) >= strtotime($wp_last_modified)) || ($client_etag == $wp_etag)) ) {
 		if ( preg_match('/cgi/',php_sapi_name()) ) {
 			header('Status: 304 Not Modified');
 			echo "\r\n\r\n";