diff --git a/tools/pip-requires b/tools/pip-requires
index 96315d00..0d5792c6 100644
--- a/tools/pip-requires
+++ b/tools/pip-requires
@@ -5,5 +5,5 @@ netaddr
 oslo.config
 paramiko
 routes
-six
+six>=1.4.0
 webob>=1.0.8