diff --git a/tox.ini b/tox.ini
index 0e97e6c8e..43abbec2c 100644
--- a/tox.ini
+++ b/tox.ini
@@ -18,6 +18,7 @@ passenv =
     HOME
     TEST_AGENTS
     LXD_DIR
+# FIXME would it be easier to `pip install -e .`?
 deps =
     macaroonbakery
     toposort
@@ -32,6 +33,7 @@ deps =
     hvac
     packaging
     setuptools
+    backports.strenum
 
 [testenv:docs]
 deps =