diff --git a/setup.py b/setup.py
index 0548d66..ec7b23f 100644
--- a/setup.py
+++ b/setup.py
@@ -68,7 +68,8 @@ setup(
     keywords=['django', 'cas', 'cas3', 'server', 'sso', 'single sign-on', 'authentication', 'auth'],
     install_requires=[
         django, 'requests >= 2.4', 'requests_futures >= 0.9.5',
-        'django-picklefield >= 0.3.1', django_bootstrap3, 'lxml >= 3.4'
+        'django-picklefield >= 0.3.1', django_bootstrap3, 'lxml >= 3.4',
+        'six >= 1'
     ],
     url="https://github.com/nitmir/django-cas-server",
     download_url="https://github.com/nitmir/django-cas-server/releases",