diff -r -u drupal-5.13.orig/includes/bootstrap.inc drupal-5.13/includes/bootstrap.inc
--- drupal-5.13.orig/includes/bootstrap.inc	2008-12-11 09:04:00 -0600
+++ drupal-5.13/includes/bootstrap.inc	2008-12-16 13:32:30 -0600
@@ -241,7 +241,7 @@
  */
 function drupal_valid_http_host() {
   $_SERVER['HTTP_HOST'] = strtolower($_SERVER['HTTP_HOST']);
-  return preg_match('/^\[?(?:[a-z0-9-:\]_]+\.?)+$/', $_SERVER['HTTP_HOST']);
+  return empty($_SERVER['HTTP_HOST']) || preg_match('/^\[?(?:[a-z0-9-:\]_]+\.?)+$/', $_SERVER['HTTP_HOST']);
 }
 
 /**