Problem updating from 11.4 to 12.whatever

I've successfully updated FreeBSD systems for many revisions, but
this one isn't working.  I'm following the instructions here:

https://www.cyberciti.biz/open-source/freebsd-12-released-here-is-how-to-upgrade-freebsd/

After a reboot, I run freebsd-update install as requested.
All I'm seeing is:

bus error (core dumped)
ld-elf.so.1: /lib/libmd.so.6: invalid file format

And of course nothing works because all the libs and binaries are
mismatched.
I can't even kill the stupid process because I only have one xterm with root
permissions and of course su and sudo are broken.

Now what?

Thanks,
Bill Dudley