Can't mount ramdisk root

I'm trying to get a machine installed and booted the SYSNBSD via rbootd.
Unfortunately it won't boot into ramdisk/sysinst:

Kernelized RAIDframe activated                            
sd0: no disk label            
boot device: ie0  
root on ie0     
nfs_boot: trying DHCP/BOOTP
[..]
no file system for ie0                                 
cannot mount root, error = 79
root device (default ie0): ? 
use one of: ie0 cd0[a-p] sd0[a-p] md0[a-p] ddb halt reboot
root device (default ie0): md0                            
dump device (default md0b):   
file system (default generic): 
root on md0a dumps on md0b     
vfs_mountroot: can't open root device

Should this work?
I can add the missing NFS root filesystem, but I expected a ramdisk with
sysinst inside the SYSNBSD kernel.

Martin