=== modified file 'mail/mail_thread.py'
--- mail/mail_thread.py	2014-02-06 12:01:09 +0000
+++ mail/mail_thread.py	2014-02-06 13:46:51 +0000
@@ -1195,6 +1195,8 @@
         """ Add partners to the records followers. """
         if context is None:
             context = {}
+        if not partner_ids:
+            return True
 
         mail_followers_obj = self.pool.get('mail.followers')
         subtype_obj = self.pool.get('mail.message.subtype')
@@ -1253,6 +1255,8 @@
 
     def message_unsubscribe(self, cr, uid, ids, partner_ids, context=None):
         """ Remove partners from the records followers. """
+        if not partner_ids:
+            return True
         user_pid = self.pool.get('res.users').read(cr, uid, uid, ['partner_id'], context=context)['partner_id'][0]
         if set(partner_ids) == set([user_pid]):
             self.check_access_rights(cr, uid, 'read')