Index: wp-includes/ms-settings.php
===================================================================
--- wp-includes/ms-settings.php	(revision 23102)
+++ wp-includes/ms-settings.php	(working copy)
@@ -128,6 +128,8 @@
 $table_prefix = $wpdb->get_blog_prefix();
 $_wp_switched_stack = array();
 $switched = false;
+if ( ! isset( $base ) )
+	$base = $current_site->path;
 
 // need to init cache again after blog_id is set
 wp_start_object_cache();