posummit: [error] 'dict' object has no attribute 'add'

Hi there,
I'm trying to set up posummit on my system, but every time I try to run `posummit scripts/messages.summit ga gather --create --force`, I get the error "posummit: [error]'dict' object has no attribute 'add'".

I've followed the instructions in https://techbase.kde.org/Localization/Workflows/PO_Summit and I'm using the most recent commit to the KDE Invent Pology repository.
Has anyone else run into this and knows how to fix it/if its a bug?
Thanks,
Charlotte