diff --git a/addons/emails/email_message.py b/addons/emails/email_message.py
index 9e00bcc6f3f..f9ce1ffee4a 100644
--- a/addons/emails/email_message.py
+++ b/addons/emails/email_message.py
@@ -238,7 +238,7 @@ class email_message(osv.osv):
                                  _("Error sending mail: %s") % e)
 
     def email_send(cr, uid, email_from, email_to, subject, body, model=False, email_cc=None, email_bcc=None, reply_to=False, attach=None,
-            openobject_id=False, debug=False, subtype='plain', x_headers=None, priority='3'):
+            openobject_id=False, debug=False, subtype='plain', x_headers={}, priority='3'):
         attachment_obj = self.pool.get('ir.attachment')
         msg_vals = {
                 'name': subject,
@@ -253,7 +253,7 @@ class email_message(osv.osv):
                 'reply_to': reply_to or '',
                 'message_id': openobject_id,
                 'sub_type': subtype or '',
-                'headers': x_headers or '',
+                'headers': x_headers or {},
                 'priority': priority,
                 'debug': debug,
                 'folder': 'outbox',
@@ -300,7 +300,7 @@ class email_message(osv.osv):
                     smtp_account = account_obj.browse(cr, uid, smtp_ids, context)[0]
                 tools.email_send(message.email_from, message.email_to, message.name, message.message, email_cc=message.email_cc,
                         email_bcc=message.email_bcc, reply_to=message.reply_to, attach=attachments, openobject_id=message.message_id,
-                        subtype=message.sub_type, x_headers=message.headers, priority=message.priority, debug=message.debug,
+                        subtype=message.sub_type, x_headers=message.headers or {}, priority=message.priority, debug=message.debug,
                         smtp_email_from=smtp_account and smtp_account.email_id or None, smtp_server=smtp_account and smtp_account.smtpserver or None,
                         smtp_port=smtp_account and smtp_account.smtpport or None, ssl=smtp_account and smtp_account.smtpssl or False,
                         smtp_user=smtp_account and smtp_account.smtpuname or None, smtp_password=smtp_account and smtp_account.smtppass or None)