[Exim] Newbie has error trying to send mail to /dev/null

I'm rather new to exim, and I suspect this may be absurdly simple, but a
Google search didn't turn anything up in the way of assistance.

I'm just trying to spamcan mail to "webmaster@???" by sending it
to /dev/null, where "somedomain" doesn't have a website and so only
opportunistic spam is going to be seen at the "webmaster" address. I have
an alias file for the domain, and it's working properly for other addresses
(both local ones and remote forwards) - but the /dev/null entry just
generates this in the log:

2002-10-17 07:03:59 /dev/null <webmaster@[somedomain].com>
D=virtual_aliases defer (-31): file_transport unset in virtual_aliases driver

I realize my problem's in the transports section exim.conf, but I don't
know where to start. I thought /dev/null was recognized by exim as a
special case and didn't take any extra setup steps to make it work...

Let me know what else I need to post of my config files, alias file, etc.

thanks...David