diff --git a/tox.ini b/tox.ini
index 03f47e3..85a4fcc 100644
--- a/tox.ini
+++ b/tox.ini
@@ -15,6 +15,7 @@ deps =
     click7: click>=7.0,<8.0
     click8: click>=8.0,<9.0
     click8-async: asyncclick>=8.0,<9.0
+    defusedxml
 commands =
     python -m pytest --cov {toxinidir}/sphinx_click {posargs}
 pip_pre =