diff --git a/tox.ini b/tox.ini
index c400a2a1..157a7df3 100644
--- a/tox.ini
+++ b/tox.ini
@@ -17,7 +17,7 @@ commands=
     nosetests -v --with-coverage --cover-package=isbnlib --cover-min-percentage=90
 
 [testenv:checkers]
-basepython=python3.7
+basepython=python3.6
 deps=
     flake8
     flake8-bugbear