diff --git a/addons/test_mass_mailing/tests/test_mailing_test.py b/addons/test_mass_mailing/tests/test_mailing_test.py
index 33c0419af07..d2bd2a96449 100644
--- a/addons/test_mass_mailing/tests/test_mailing_test.py
+++ b/addons/test_mass_mailing/tests/test_mailing_test.py
@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
+import lxml.html
 
 from odoo.addons.test_mass_mailing.tests.common import TestMassMailCommon
 from odoo.tests.common import users
@@ -28,6 +29,16 @@ class TestMailingTest(TestMassMailCommon):
         with self.mock_mail_gateway():
             mailing_test.send_mail_test()
 
+        # not great but matches send_mail_test, maybe that should be a method
+        # on mailing_test?
+        record = self.env[mailing.mailing_model_real].search([], limit=1)
+        first_child = lxml.html.fromstring(self._mails.pop()['body']).xpath('//body/*[1]')[0]
+        self.assertEqual(first_child.tag, 'div')
+        self.assertIn('display:none', first_child.get('style'),
+                      "the preview node should be hidden")
+        self.assertEqual(first_child.text.strip(), "Preview " + record.name,
+                         "the preview node should contain the preview text")
+
         # Test if bad jinja in the subject raises an error
         mailing.write({'subject': 'Subject ${object.name_id.id}'})
         with self.mock_mail_gateway(), self.assertRaises(Exception):
diff --git a/odoo/tools/mail.py b/odoo/tools/mail.py
index bbc8a660451..bfbfddea275 100644
--- a/odoo/tools/mail.py
+++ b/odoo/tools/mail.py
@@ -433,14 +433,11 @@ def append_content_to_html(html, content, plaintext=True, preserve=False, contai
 
 def prepend_html_content(html_body, html_content):
     """Prepend some HTML content at the beginning of an other HTML content."""
-    html_content = re.sub(r'(?i)(</?(?:html|body|head|!\s*DOCTYPE)[^>]*>)', '', html_content)
+    html_content = type(html_content)(re.sub(r'(?i)(</?(?:html|body|head|!\s*DOCTYPE)[^>]*>)', '', html_content))
     html_content = html_content.strip()
 
-    insert_index = next(re.finditer(r'<body[^>]*>', html_body), None)
-    if insert_index is None:
-        insert_index = next(re.finditer(r'<html[^>]*>', html_body), None)
-
-    insert_index = insert_index.end() if insert_index else 0
+    body_match = re.search(r'<body[^>]*>', html_body) or re.search(r'<html[^>]*>', html_body)
+    insert_index = body_match.end() if body_match else 0
 
     return html_body[:insert_index] + html_content + html_body[insert_index:]