diff --git a/requirements.txt b/requirements.txt
index 7bc399b1..adbbcb27 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -8,3 +8,4 @@ premailer==2.9.6
 psycopg2==2.6.1
 Pygments==2.0.2
 requests==2.8.1
+six==1.10.0