diff --git a/test-requirements.txt b/test-requirements.txt
index 83877c2..6d929e8 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -4,4 +4,5 @@
 
 coverage
 nose
+six>=1.9.0
 sphinx>=1.1.2,!=1.2.0,!=1.3b1,<1.3  # BSD