diff --git a/wp-includes/ms-default-constants.php b/wp-includes/ms-default-constants.php
index 75ea1e6e7..49746ef04 100644
--- a/wp-includes/ms-default-constants.php
+++ b/wp-includes/ms-default-constants.php
@@ -61,8 +61,12 @@ function ms_cookie_constants(  ) {
 	/**
 	 * @since 2.0.0
 	 */
-	if ( !defined('COOKIE_DOMAIN') )
-		define('COOKIE_DOMAIN', '.' . $current_site->cookie_domain);
+	if ( !defined('COOKIE_DOMAIN') ) {
+		if ( isset( $current_site->cookie_domain ) )
+			define('COOKIE_DOMAIN', '.' . $current_site->cookie_domain);
+		else
+			define('COOKIE_DOMAIN', '.' . $current_site->domain);
+	}
 }
 
 /**
@@ -85,4 +89,4 @@ function ms_file_constants(  ) {
 	if ( !defined( 'WPMU_ACCEL_REDIRECT' ) )
 		define( 'WPMU_ACCEL_REDIRECT', false );
 }
-?>
\ No newline at end of file
+?>