Dual-booting an EFI/GPT system?

I'm trying to put an old intel mac mini to some test use and I need to
install both a linux and a netbsd on it.  I haven't installed a
dual-boot netbsd system in quite a while ("use whole disk" ftw) and
sepcificially not with GPT.

I've installed Ubuntu with plenty of free space left for NetBSD.  I
start netbsd installer, add GPT partitions for netbsd root, swap and
home.  When I proceed, installtion fails immediately with

    gpt biosboot -b ${nbroot_offset} wd0

What am I doint wrong?  Should I instead use single GPT partition for
the whole NetBSD installtion and partition its insides with netbsd
disklabel?

TIA.

-uwe