diff --git a/pyproject.toml b/pyproject.toml
index c469a16be48..0697a9f4fe1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -113,6 +113,7 @@ namespace_packages = true
 # globally, then turn back on in spack and spack submodules
 ignore_errors = true
 ignore_missing_imports = true
+check_untyped_defs = true
 
   [[tool.mypy.overrides]]
   module = 'spack.*'