diff --git a/tox.ini b/tox.ini
index 2cf792f14..7fcf2581f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -54,6 +54,7 @@ deps =
     flake8 == 3.4.1
     flake8-isort == 2.2.2
     flake8-quotes == 0.11.0
+    isort < 4.3.5
 
 [testenv:check_migrated]
 setenv =