diff --git a/pyproject.toml b/pyproject.toml
index a7a0125..3c04fae 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -88,6 +88,7 @@ testpaths = ["tests"]
 [tool.mypy]
 python_version = "3.8"
 strict = true
+explicit_package_bases = true
 
 [tool.coverage.run]
 source = ["asphalt.serialization"]