diff --git a/docs/requirements.txt b/docs/requirements.txt
index ffe2fce498955b628014618b28c6bcf152466a4a..ebd8d0846788f6da0c0613bd83290105e260929b 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1 +1,2 @@
 six
+six