diff --git a/doc/requirements.txt b/doc/requirements.txt
index e9992a8db2..e5abe08f4f 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,5 +1,5 @@
 -r ../src/requirements.txt
-sphinx
+sphinx==1.6.*
 sphinx-rtd-theme
 sphinxcontrib-httpdomain
 sphinxcontrib-images