Applying the patch included there fixed it.
Apologies, turns out that this is a duplicate of
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=838288
- Bug#873505: mailman: Runner crashes when proces... Pete Donnell
- Bug#873505: Acknowledgement (mailman: Runn... Pete Donnell
- Bug#873505: [Pkg-mailman-hackers] Bug#... Thijs Kinkhorst
- Processed: Re: [Pkg-mailman-hacker... Debian Bug Tracking System
