diff --git a/duties.py b/duties.py
index b4ef4474..51cef860 100644
--- a/duties.py
+++ b/duties.py
@@ -114,6 +114,7 @@ def check_types(ctx: Context) -> None:
     Parameters:
         ctx: The context instance (passed automatically).
     """
+    os.environ["MYPYPATH"] = "src"
     ctx.run(
         mypy.run(*PY_SRC_LIST, config_file="config/mypy.ini"),
         title=pyprefix("Type-checking"),