locally built netboot image errors out at partman stage

Hi,

when I boot a qemu machine from 
https://d-i.debian.org/daily-images/amd64/daily/netboot/mini.iso, 
everything is fine. I can proceed until partman comes up (I abort then, 
partman is what interests me).

When I check out 
https://salsa.debian.org/installer-team/debian-installer.git, do make 
reallyclean, make build_netboot and boot from that mini.iso, at the 
stage where partman is supposed to come up, I get a red screen saying 
that partitioning disks failed.

The log then says:
Jul 15 09:03:01 main-menu[259]: INFO: Menu item 'partman-base' selected
Jul 15 09:03:01 main-menu[259]: WARNING **: Deep recursion configuring package partman-base (dep loop?)
Jul 15 09:03:01 main-menu[259]: WARNING **: Menu item 'partman-base' failed.

Why does my image behave differently? Any ideas what might be going 
wrong here?

Greetings
Marc