Django rules object permissions

I am trying to use django rules to configure object permissions in django and the django admin interface.

Now when I add permission rules, they will always only be called with the first param, but the object is always None.

For example I if I would create this predicate:

@rules.predicate
def is_book_author(user, book):
    return book.author == user

And then add it to the django permission set:

rules.add_perm('books.view_book', is_book_author)

Now when I log into the admin interface with a user, then the is_book_author will be called with the user and None. It will be called multiple times (once per object), but the object ist always None.

I am using rules 2.0.0 with django 2.1.1 and python 3.7.

Any ideas if I'm doing something wrong or how to configure django to call the predicate with the individual object?

from Django rules object permissions