panic: cannot find root vnode

After running in to the panic mentioned here:

http://mail-index.netbsd.org/port-evbmips/2016/11/08/msg000328.html

which corresponds to this:

http://gnats.netbsd.org/50766

I tried to update to today's sources with a assertions skipped, but it 
seems that neither this kernel, nor a kernel from a few days ago (from 
nyftp.netbsd.org) can mount the root filesystem:


boot device: sd0
sd0: fabricating a geometry
root on sd0f dumps on sd0b
mountroot: trying ffs...
root file system type: ffs
panic: cannot find root vnode, error=2
kernel: breakpoint trap
Stopped in pid 0.1 (system) at  netbsd:cpu_Debugger+0x4:        jr      ra
                 bdslot: nop


Switching back to the kernel from the end of October does still mount the 
filesystem, as does a macppc system, so I know the filesystem is fine.

It seems that there are problems trying to create an fdisk / MBR partition 
scheme from a little endian system (amd64), but that's a completely 
separate problem.

Anyone else?

John