diff --git a/requirements.txt b/requirements.txt
index 9f8c683..93f0007 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -4,6 +4,7 @@
 
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 
+six>=1.10.0 # MIT
 tempest>=17.1.0 # Apache-2.0
 cryptography!=2.0,>=1.9 # BSD/Apache-2.0