diff --git a/python/tox.ini b/python/tox.ini
index eeb4d2b88..946412cea 100644
--- a/python/tox.ini
+++ b/python/tox.ini
@@ -46,10 +46,9 @@ commands =
 [testenv:doc]
 changedir=doc
 deps =
-    # With Python 3.7 and Sphinx 3.5.0, it throws an exception.
-    # Pin the version to the working one.
+    # Pin dependencies to the versions which actually work with Python 3.5.
     sphinx==3.4.3
-    breathe
+    breathe==4.28.0
 commands =
     sphinx-build -Q -w toxdoc-warnings.log -b html . _build/html