Comment 1 for bug 1851029

Hmm, yes, it's probably worth reconsidering that assertion. In the end, what you're asking for is to discard the entire "document". That's a valid thing to do, and it should not run into an AssertionError.
OTOH, what should it return in such a case?
Maybe it could be enough to remove all content from the tag (children and inner text), and then still return it?