synth 1.52 not working - tmpfs issue

Although I have configured synth to not use tmpfs, it still uses it:

# synth upgrade-system
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
Can somebody please tell me what went wrong here?
Check if that filesystem isn't already mounted. A previous run may have left it behind.
synth always uses tmpfs in some capacity.
The "use tmpfs" setting refers to using it for the build area and the localbase, which can be huge.

What's the cause of that error message? In other words, what is preventing that command from working? (directory doesn't exist, already mounted, disk full, etc)
I think that I have disabled tmpfs in the kernel somehow. Is it the "device md" (memory disk) that my kernel would need?
see man 5 tmpfs

You might be able to dynamically kload it, but I'm not sure why you would ever not want tmpfs support.
Is this error also produced by including the following in /etc/rc.conf?