diff --git a/tox.ini b/tox.ini
index d448b68..fd77d2d 100644
--- a/tox.ini
+++ b/tox.ini
@@ -85,7 +85,8 @@ deps = -r{toxinidir}/requirements.txt
 [testenv:pep8]
 basepython = python3
 deps = flake8==3.9.2
-       charm-tools==2.8.3
+       PyYAML==6.0.1
+       charm-tools==2.8.6
 commands = flake8 {posargs} hooks unit_tests tests actions lib files
            charm-proof