try:
        from pydantic_core.core_schema import (
            with_info_plain_validator_function as with_info_plain_validator_function,
        )
    except ImportError:  # pragma: no cover
        from pydantic_core.core_schema import (
            general_plain_validator_function as with_info_plain_validator_function,  # noqa: F401
        )

    Required = PydanticUndefined

    app = FastAPI()
    with pytest.raises(FastAPIError) as e:

        @app.get("/")
        def read_root() -> Union[Response, None]:
            return Response(content="Foo")  # pragma: no cover

    assert "valid Pydantic field type" in e.value.args[0]
    assert "parameter response_model=None" in e.value.args[0]


def test_openapi_schema():
    response = client.get("/openapi.json")