Using Request mixed Forms and Files with Annotation and optional fields

I'd like to post mixed form fields and upload files to a FastAPI endpoint. The FastAPI documentation here states that you can mix params using Annotated and the python-multipart library e.g.

@router.post("/upload")
async def upload_contents(
    an_int: Annotated[int, Form()],
    a_string: Annotated[str, Form()], 
    some_files: Annotated[list[UploadFile], File()]
):

However if I want to make posting of a_string optional, I have not found a way to make this work.

I have tried things like:

@router.post("/upload")
async def upload_contents(
    an_int: Annotated[int, Form()],
    a_string: Annotated[Union[str, None], Form()], 
    some_files: Annotated[list[UploadFile], File()]
):

And also on @flakes suggestion:

@router.post("/upload")
async def upload_contents(
    an_int: Annotated[int, Form()],
    a_string: Optional[Annotated[str], Form()]], 
    some_files: Annotated[list[UploadFile], File()]
):

While these run, the server is still requesting I provide the a_string parameter. Any guidance would be gratefully received.

It was discussed in this issue.

Basically, if the default value is set in Form(), the parameter will be optional.

@router.post("/upload")
async def upload_contents(
    an_int: Annotated[int, Form()],
    some_files: Annotated[List[UploadFile], File()],
    a_string: Union[str, None] = Form(default=None)
):
    return True

It seems that using Annotated[Union[str, None], Form(default=None)] isn't working and will return the following error

AssertionError: `Form` default value cannot be set in `Annotated` for 'a_string'. Set the default value with `=` instead.