Repository.merge_file_from_index - can't take a None as an argument

This is probably more of a libgit2 question but given that I am working on pygit2, I better start the conversation over here.

I am working on writing unit tests for Repository.merge_file_from_index. I want to add a test for a deleted file. The signature of the method provides for None as a valid argument:

    def merge_file_from_index(
        self,
        ancestor: typing.Union[None, IndexEntry],
        ours: typing.Union[None, IndexEntry],
        theirs: typing.Union[None, IndexEntry],
    ) -> str:

So I have this:

def test_merge_file_from_index(testrepo):
    hello_txt = testrepo.index["hello.txt"]

    # deleting the file on theirs
    res = testrepo.merge_file_from_index(hello_txt, hello_txt, None)
    assert res == ""

And I am getting this when I run the test:

>       raise GitError(message)
E       _pygit2.GitError: invalid argument: 'theirs'

Am I wrong in my understanding from the signature that None is a correct value?

Read pygit2/repository.py around line 682 and run the shown test for Repository.merge_file_from_index. Check whether the implementation accepts None for the theirs argument as the signature indicates; done means the behavior and test agree on the deleted-file case.