diff --git a/ckan/lib/mailer.py b/ckan/lib/mailer.py
index d40a0d5b5e9..22d178858ec 100644
--- a/ckan/lib/mailer.py
+++ b/ckan/lib/mailer.py
@@ -28,7 +28,11 @@ class MailerException(Exception):
 
 def _mail_recipient(recipient_name, recipient_email,
                     sender_name, sender_url, subject,
-                    body, headers={}):
+                    body, headers=None):
+
+    if not headers:
+        headers = {}
+
     mail_from = config.get('smtp.mail_from')
     msg = MIMEText(body.encode('utf-8'), 'plain', 'utf-8')
     for k, v in headers.items():