diff --git a/requirements.txt b/requirements.txt
index 79ee525..e0a5e6e 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -10,3 +10,4 @@ alembic>=0.6.4
 PyYAML>=3.1.0
 voluptuous>=0.7
 ply>=3.4
+six