diff --git a/wp-includes/load.php b/wp-includes/load.php
index 37fa7d0840..5489f4a62f 100644
--- a/wp-includes/load.php
+++ b/wp-includes/load.php
@@ -647,7 +647,7 @@ function is_multisite() {
 	if ( defined( 'MULTISITE' ) )
 		return MULTISITE;
 
-	if ( defined( 'VHOST' ) || defined( 'SUNRISE' ) )
+	if ( defined( 'SUBDOMAIN_INSTALL' ) || defined( 'VHOST' ) || defined( 'SUNRISE' ) )
 		return true;
 
 	return false;