diff --git a/setup.cfg b/setup.cfg
index f8be2cf..a03dda3 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -42,7 +42,7 @@ deps =
 commands =
     black --check --diff src tests
     flake8 src tests
-    mypy src tests
+    mypy --disallow-untyped-defs src tests
     pytest tests
 
 [flake8]
diff --git a/src/basicpy/__init__.py b/src/basicpy/__init__.py
index 5db74f3..9169e98 100644
--- a/src/basicpy/__init__.py
+++ b/src/basicpy/__init__.py
@@ -1,2 +1,2 @@
-def main():
+def main() -> None:
     print("Hello World")
diff --git a/tests/test_basicpy.py b/tests/test_basicpy.py
index 94e5242..a3ba9dc 100644
--- a/tests/test_basicpy.py
+++ b/tests/test_basicpy.py
@@ -1,2 +1,2 @@
-def test_basicpy():
+def test_basicpy() -> None:
     pass