diff --git a/docs/requirements-docs.txt b/docs/requirements-docs.txt
index 3ba864a..2622ddf 100644
--- a/docs/requirements-docs.txt
+++ b/docs/requirements-docs.txt
@@ -7,4 +7,4 @@
 sphinxcontrib-plantuml
 sphinxcontrib-redoc
 sphinx-tabs
-
+six