diff --git a/doc/requirements.txt b/doc/requirements.txt
index 09906c15..8de6c6a8 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,2 +1,3 @@
 exhale
-recommonmark
\ No newline at end of file
+recommonmark
+sphinx==3.1.1