userland breakage, zfs ?

Just updated two machines in the fbsd cluser, buildworld/kernel,
installkernel, reboot into single user.

zfs from oldworld (pre installworld) doesn't work and segfaults in new
and magical ways.

Trying to mount root from zfs:zroot []...
Enter full pathname of shell or RETURN for /bin/sh: 
Cannot read termcap database;
using dumb terminal settings.
# zfs set readonly=off zroot
internal error: pInvalid argumentid
 19 (zfs), uid 0: exited on signal 6
Abort trap


I guess, I could multiuser the box on old kernel and installworld from
multiuser, but this could lead to disaster.  I see no indication of ABI
breakage of this kind in UPDATING, so ... any ideas?

sean