Merged standard Dict and TypedDict to form a TypedDict in Python?

I'm trying to use code like this to create a TypedDict which is a superset of another TypedDict:

from typing import TypedDict


class Base(TypedDict):
    a: str


class Extend(Base):
    b: str


base_instance: Base = {"a": "string!"}

extend_instance: Extend = {**base_instance, "b": "another string"}

But this emits a diagnostic warning about extend_instance: "Expression of type "dict[str, object | str]" cannot be assigned to declared type "Extend""

It's not clear to me why the values in extend_instance are assumed to be object | str, rather than just str.

Is there something I can do to annotate this better?

Thanks in advance.