diff --git a/python/tox.ini b/python/tox.ini
index 9417c5544..c33eff722 100644
--- a/python/tox.ini
+++ b/python/tox.ini
@@ -47,7 +47,9 @@ commands =
 [testenv:doc]
 changedir=doc
 deps =
-    sphinx
+    # With Python 3.7 and Sphinx 3.5.0, it throws an exception.
+    # Pin the version to the working one.
+    sphinx==3.4.3
     breathe
 commands =
     sphinx-build -Q -w toxdoc-warnings.log -b html . _build/html