diff --git a/requirements.txt b/requirements.txt
index 2dc6330..ba446a2 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -5,3 +5,4 @@
 pbr<2.0,>=1.4
 Babel>=1.3
 PyYAML>=3.1.0
+six>=1.9.0