diff --git a/Makefile b/Makefile
index 053e782..745e3c9 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,7 @@ SHELL=/bin/bash
 
 lint:
 	flake8
-	mypy requests_http_signature
+	mypy --check-untyped-defs requests_http_signature
 
 test: lint
 	python ./test/test.py -v