diff --git a/tox.ini b/tox.ini
index 731094b5d..3c398dc3b 100644
--- a/tox.ini
+++ b/tox.ini
@@ -134,7 +134,7 @@ commands = /bin/sh -c "flake8 synapse tests scripts scripts-dev scripts/hash_pas
 [testenv:check_isort]
 skip_install = True
 deps = isort
-commands = /bin/sh -c "isort -c -sp setup.cfg -rc synapse tests"
+commands = /bin/sh -c "isort -c -df -sp setup.cfg -rc synapse tests"
 
 [testenv:check-newsfragment]
 skip_install = True
@@ -150,4 +150,4 @@ deps =
     codecov
 commands =
     coverage combine
-    codecov -X gcov
\ No newline at end of file
+    codecov -X gcov