Read the instructions carefully.
This one was rather obvious and therefore difficult to troubleshoot.
I did a binary upgrade of FreeBSD via
freebsd-update, rebooted and then tried
portmaster -a. This resulted in:
The value of DISTDIR cannot be empty.
What I should have done was run
freebsd-update install again after the reboot. The instructions clearly stated that. There is an argument to be made for having something resembling an attention span.