Index: app/models/issue.rb
===================================================================
--- app/models/issue.rb	(revision 14044)
+++ app/models/issue.rb	(working copy)
@@ -824,7 +824,7 @@
     # assigned_to_id_was is reset before after_save callbacks
     user_id = @previous_assigned_to_id || assigned_to_id_was
     if user_id && user_id != assigned_to_id
-      @assigned_to_was ||= User.find_by_id(user_id)
+      @assigned_to_was ||= User.find_by_id(user_id) || Group.find_by_id(user_id)
     end
   end