Index: wp-includes/ms-default-constants.php
===================================================================
--- wp-includes/ms-default-constants.php	(revision 23001)
+++ wp-includes/ms-default-constants.php	(working copy)
@@ -56,8 +56,12 @@
 	/**
 	 * @since 1.5.0
 	 */
-	if ( !defined( 'SITECOOKIEPATH' ) )
-		define( 'SITECOOKIEPATH', $current_site->path );
+	if ( !defined( 'SITECOOKIEPATH' ) ) {
+		if ( !is_subdomain_install() )
+			define( 'SITECOOKIEPATH', $current_site->path );
+		else
+			define( 'SITECOOKIEPATH', trailingslashit( parse_url( get_option( 'siteurl' ), PHP_URL_PATH ) ) );
+	}
 
 	/**
 	 * @since 2.6.0