diff -r -u drupal-5.14.orig/includes/bootstrap.inc drupal-5.14/includes/bootstrap.inc
--- drupal-5.14.orig/includes/bootstrap.inc	2008-12-12 09:56:15.000000000 -0600
+++ drupal-5.14/includes/bootstrap.inc	2008-12-12 09:56:22.000000000 -0600
@@ -240,6 +240,8 @@
  *  TRUE if only containing valid characters, or FALSE otherwise.
  */
 function drupal_valid_http_host() {
+  if (empty($_SERVER['HTTP_HOST'])) return true;
+
   $_SERVER['HTTP_HOST'] = strtolower($_SERVER['HTTP_HOST']);
   return preg_match('/^\[?(?:[a-z0-9-:\]_]+\.?)+$/', $_SERVER['HTTP_HOST']);
 }