diff --git a/app/models/mailer.rb b/app/models/mailer.rb
index 63fbe6c..20ce65b 100644
--- a/app/models/mailer.rb
+++ b/app/models/mailer.rb
@@ -381,7 +381,7 @@ class Mailer < ActionMailer::Base
 
   # Appends a Redmine header field (name is prepended with 'X-Redmine-')
   def redmine_headers(h)
-    h.each { |k,v| headers["X-Redmine-#{k}"] = v }
+    h.each { |k,v| headers["X-Redmine-#{k}"] = v.to_s }
   end
 
   # Overrides the create_mail method