[exim] once in auto_reply transport?

i'm trying to get the once option working in an autoreply transport,
and i don't see what the problem is here.

# not whitelisted
not_on_whitelist_reply:
    driver = autoreply
    file = /etc/exim/not_on_whitelist_reply.txt
    once = /etc/exim/not_whitelisted_senders.db
    once_file_size = 100000

the .db file exists, is zero length, owned by exim, permissions
777, and exim isn't putting anything into it. i'm not seeing
anything in the log files about the file, i can certainly
see the transport executing properly (except for the file
update.)

exim-4.66, fedora core 5 linux, with whatever db library the
install defaults to.

any ideas?

thanks,
richard