What's the problem with validator.optional and a default string?

[hf-kklein]

src\maus\models\anwendungshandbuch.py:156: error: Argument "validator" to "field" has incompatible type "Callable[[Any, Attribute[Optional[str]], Optional[str]], Any]"; expected "Union[Callable[[Any, Attribute[str], str], Any], Sequence[Callable[[Any, Attribute[str], str], Any]], None]" [arg-type]