mount -t tmpfs tmpfs fails

I was attempting to run 'synth status' on my 12-current (drm-next) system
and I got this error:

root_at_tavion:0 /> synth status
Querying system about current package installations.
Stand by, comparing installed packages against the ports tree.

raised REPLICANT.SCENARIO_UNEXPECTED : /sbin/mount -t tmpfs tmpfs /usr/obj/synth-live/SL09 => failed with code 1

So then I attempted just to do a mount of tmpfs command and got this:

root_at_tavion:0 /> mount -t tmpfs x /tmp/xxx  
mount: x: Operation not supported by device

synth used to work on this system prior to my recent upgrade to the latest
drm-next branch (cloned from github and build from source).

root_at_tavion:1 /> uname -a
FreeBSD tavion.austin.ibm.com 12.0-CURRENT FreeBSD 12.0-CURRENT #1 da5f90154f1(drm-next): Tue Jun 13 16:58:52 CDT 2017     bob_at_tavion.austin.ibm.com:/usr/obj/usr/freebsd-base-graphics/sys/TAVION  amd64

Any suggestions on what I should be looking for (or fixing)?

Thanks,
Bob

-- 
Bob Willcox    | Lawsuit, n.: A machine which you go into as a pig and
bob_at_immure.com |    come out as a sausage.
Austin, TX     |    -- Ambrose Bierce