Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 23106)
+++ wp-includes/ms-functions.php	(working copy)
@@ -540,8 +540,10 @@
  * @return array Contains the new site data and error messages.
  */
 function wpmu_validate_blog_signup($blogname, $blog_title, $user = '') {
-	global $wpdb, $domain, $base, $current_site;
+	global $wpdb, $domain, $current_site;
 
+	$base = $current_site->path;
+
 	$blog_title = strip_tags( $blog_title );
 	$blog_title = substr( $blog_title, 0, 50 );