diff --git a/eng/tox/tox.ini b/eng/tox/tox.ini
index 5ff983a5a6da..c4c867fc5f00 100644
--- a/eng/tox/tox.ini
+++ b/eng/tox/tox.ini
@@ -152,6 +152,7 @@ deps =
   sphinx==3.5.4
   sphinx_rtd_theme==0.5.0
   recommonmark==0.6.0
+  mistune<2.0.0
   m2r==0.2.1
 commands =
   {envbindir}/python {toxinidir}/../../../eng/tox/prep_sphinx_env.py -d {distdir} -t {toxinidir}