Reported by an ISFDB moderator:
If a chapbook ends up in a series information somehow and a series number of "0", the series name can be cleaned up easily but the series number is greyed out and you cannot save the chapbook in any way or form to fix it. The only way I discovered was a temporary change of type on a first edit and then a second save to chapbook with all values cleared out. I am not sure if it happens for all chapbooks or only when the series number is "0".
Anonymous
Fixed in edit/edittitle.py, installed in SVN 283 on 2018-12-25. Closing the bug.