Index: includes/bootstrap.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/bootstrap.inc,v
retrieving revision 1.257
diff -u -p -r1.257 bootstrap.inc
--- includes/bootstrap.inc	3 Dec 2008 14:51:53 -0000	1.257
+++ includes/bootstrap.inc	16 Dec 2008 20:47:19 -0000
@@ -417,7 +417,7 @@ function drupal_initialize_variables() {
  */
 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']);
 }
 
 /**
Index: modules/simpletest/tests/bootstrap.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/tests/bootstrap.test,v
retrieving revision 1.9
diff -u -p -r1.9 bootstrap.test
--- modules/simpletest/tests/bootstrap.test	3 Dec 2008 14:51:53 -0000	1.9
+++ modules/simpletest/tests/bootstrap.test	16 Dec 2008 20:47:19 -0000
@@ -68,7 +68,7 @@ class BootstrapIPAddressTestCase extends
     // Cluster environment.
     $_SERVER['HTTP_X_CLUSTER_CLIENT_IP'] = $this->cluster_ip;
     $this->assertTrue(
-      ip_address(true) == $this->cluster_ip,
+      ip_address(TRUE) == $this->cluster_ip,
       t('Cluster environment got cluster client IP')
     );
     $_SERVER['HTTP_HOST'] = 'security/.drupal.org:80';
@@ -81,6 +81,8 @@ class BootstrapIPAddressTestCase extends
     $this->assertFalse(drupal_valid_http_host(), t('HTTP_HOST with .. is invalid'));
     $_SERVER['HTTP_HOST'] = '[::1]:80'; // IPv6 loopback address
     $this->assertTrue(drupal_valid_http_host(), t('HTTP_HOST containing IPv6 loopback is valid'));
+    $_SERVER['HTTP_HOST'] = '';
+    $this->assertTrue(drupal_valid_http_host(), t('Empty HTTP_HOST is valid'));
   }
 }