def METHOD_NAME(self, chk, obj_size: int) -> None:
    """Helper to add a key into `checksum_size` while also performing
    a sanity check to ensure that, if the checksum already existed in the
    map, the size is the same.
    """
    size = self._synonyms.checksum_size.setdefault(chk, obj_size)
    assert (
        size == obj_size
    ), f"{chk} has prior size {size}, while the new size is {obj_size}"