diff --git a/_shared/project/pyproject.toml b/_shared/project/pyproject.toml
index ee6aaf1..dc3d347 100644
--- a/_shared/project/pyproject.toml
+++ b/_shared/project/pyproject.toml
@@ -181,6 +181,7 @@ pretty = true
 warn_unused_configs = true
 warn_redundant_casts = true
 warn_unused_ignores = true
+check_untyped_defs = true
 
 disable_error_code = [
     # https://mypy.readthedocs.io/en/stable/error_code_list.html#code-import-untyped