--- /usr/local/www/redmine/app/models/mailer.rb.orig	2011-12-02 22:31:14.000000000 +0000
+++ /usr/local/www/redmine/app/models/mailer.rb	2012-03-28 11:45:09.000000000 +0000
@@ -354,7 +354,7 @@
                                           :conditions => s.conditions
                                     ).group_by(&:assigned_to)
     issues_by_assignee.each do |assignee, issues|
-      deliver_reminder(assignee, issues, days) if assignee && assignee.active?
+      deliver_reminder(assignee, issues, days) if assignee.is_a?(User) && assignee.active?
     end
   end