Index: wp-includes/load.php
===================================================================
--- wp-includes/load.php	(revision 15489)
+++ wp-includes/load.php	(working copy)
@@ -595,7 +595,7 @@
 	if ( defined( 'MULTISITE' ) )
 		return MULTISITE;
 
-	if ( defined( 'VHOST' ) || defined( 'SUNRISE' ) )
+	if ( defined( 'SUBDOMAIN_INSTALL' ) || defined( 'VHOST' ) || defined( 'SUNRISE' ) )
 		return true;
 
 	return false;