diff -u b/modules/system/system.install b/modules/system/system.install
--- b/modules/system/system.install
+++ b/modules/system/system.install
@@ -3178,7 +3178,8 @@
   db_truncate('sessions')->execute();
   // Force to write again the current session to have it hashed.
   drupal_save_session(TRUE);
-  _drupal_session_write(session_id(), '');
+  global $user;
+  _drupal_session_write(session_id(), $user->session);
   drupal_save_session(FALSE);
 
 }