"ERROR: You must give at least one requirement to install" when building a snap

I’m trying to builds fades from master, and it fails with the following error (it worked just fine last release some months ago):

ERROR: You must give at least one requirement to install (maybe you meant "pip install /home/facundo/devel/reps/fades/parts/fades/python-packages"?)
Failed to run '/home/facundo/devel/reps/fades/parts/fades/install/usr/bin/python3 -m pip install --user --no-compile --no-index --find-links /home/facundo/devel/reps/fades/parts/fades/python-packages --upgrade --no-deps': Exited with code 1.

Full logs here.

Note that there’s a similar forum post here, but that is focused in when rebuilding parts (and worked around cleaning dirs, which doesn’t work for me).