diff --git a/certbot-postfix/setup.py b/certbot-postfix/setup.py
index 1f087ccc0..2fdcd46aa 100644
--- a/certbot-postfix/setup.py
+++ b/certbot-postfix/setup.py
@@ -12,6 +12,7 @@ install_requires = [
     # For pkg_resources. >=1.0 so pip resolves it to a version cryptography
     # will tolerate; see #2599:
     'setuptools>=1.0',
+    'six',
     'zope.interface',
 ]