diff --git a/docs/requirements.txt b/docs/requirements.txt
index debed2867..7c17f0c8d 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1,6 +1,6 @@
 # This is a list of python packages used to generate documentation. This file is used with pip:
 # pip install requirements.txt
 #
-sphinx
+sphinx==1.5.6
 sphinx-rtd-theme
 breathe