diff --git a/tox.ini b/tox.ini
index 08de9e6..648d30a 100644
--- a/tox.ini
+++ b/tox.ini
@@ -4,7 +4,7 @@ skipsdist = True
 [testenv]
 commands =
   python3 -m yapf --diff --recursive phile setup.py test_phile
-  python3 -m mypy .
+  python3 -m mypy --show-error-codes .
   python3 -m coverage erase
   python3 -m coverage run
   python3 -m coverage report