'postdrop' problem when using pkg_comp

On several of my machines, I did a massive package rebuild using
pkg_comp.  It worked fine on most of them, but on one I got many
messages like this:

postdrop: warning: mail_queue_enter: create file maildrop/991808.8467:
No such file or directory

I assume it had to do with invoking postfix in a chrooted environment.
However, I've never seen this before, and it didn't happen on the other
machines.  Any theories?  (Most of the machines in question run
-current, including the failing one and some of the successful ones; one
is on 3.0.1.)

                --Steve Bellovin, http://www.cs.columbia.edu/~smb