diff --git a/tox.ini b/tox.ini
index 8030bc7..041a8e0 100644
--- a/tox.ini
+++ b/tox.ini
@@ -33,7 +33,7 @@ description = invoke sphinx-build to build HTML docs
 basepython = python3.7
 deps =
     sphinx >= 3.5.1, < 4
-    m2r
+    m2r2 == 0.3.2
 ignore_errors = true
 commands =
     # https://tox.readthedocs.io/en/latest/example/documentation.html