diff --git a/tox.ini b/tox.ini
index 9466238..1f25087 100644
--- a/tox.ini
+++ b/tox.ini
@@ -36,6 +36,7 @@ deps =
   https://git.entrouvert.org/authentic.git/snapshot/authentic-main.tar.gz
   # dependency constraints for authentic
   drf39: djangorestframework>=3.9.2,<3.10
+  drf39: Markdown<3.4
   drf312: djangorestframework>=3.12,<3.13
   django<2.3
   django-tables<2.0