diff --git a/addons/mass_mailing/models/mailing.py b/addons/mass_mailing/models/mailing.py
index 21583e08f68..9014169bc47 100644
--- a/addons/mass_mailing/models/mailing.py
+++ b/addons/mass_mailing/models/mailing.py
@@ -1208,6 +1208,6 @@ class MassMailing(models.Model):
                 modified = True
 
         if modified:
-            return lxml.html.tostring(root)
+            return lxml.html.tostring(root, encoding='unicode')
 
         return body_html