Index: wp-admin/includes/schema.php
===================================================================
--- wp-admin/includes/schema.php	(revision 21249)
+++ wp-admin/includes/schema.php	(working copy)
@@ -288,7 +288,8 @@
   activation_key varchar(50) NOT NULL default '',
   meta longtext,
   KEY activation_key (activation_key),
-  KEY domain (domain)
+  KEY domain (domain),
+  KEY user_email (user_email)
 ) $charset_collate;";
 
 	switch ( $scope ) {