diff --git a/stdlib/3/types.pyi b/stdlib/3/types.pyi
index 4a0da754e..fe34809a5 100644
--- a/stdlib/3/types.pyi
+++ b/stdlib/3/types.pyi
@@ -68,7 +68,8 @@ class MappingProxyType(Mapping[_KT, _VT], Generic[_KT, _VT]):
     def __iter__(self) -> Iterator[_KT]: ...
     def __len__(self) -> int: ...
 
-class SimpleNamespace(Any): ...
+# TODO: use __getattr__ and __setattr__ instead of inheriting from Any, pending mypy#521.
+class SimpleNamespace(Any): ...  # type: ignore
 
 class GeneratorType:
     gi_code = ...  # type: CodeType