[Exim] Failing dbmnz lookup with exim-4.04

I'm (nearly) finished getting a new system setup with exim-4.04,
amavisd-new, cyrus-imap and other assorted bits. In my "local users" router,
I want to exim to look directly into cyrus' mailboxes.db file to see if
there is a mailbox for the potential local part.

In my router, I have defined:
condition = ${lookup{user.${local_part}} dbmnz
{/opt/var/imap/mailboxes.db} {yes}{no}}

This should add a "user." prefix to the local part, and look for an imap
mailbox with that name.

To see if this would work, I invoked "exim -be" and typed in the lookup
condition above (with a hardcoded local part, of course). The result is:

Failed: failed to open /opt/var/imap/mailboxes.db as a db (v3) file: Success

I _know_ this is a valid db3 file, I can dump it using db_dump and see the
contents (and they are as I expect). exim and cyrus were compiled today on
this machine, using the same db3 libraries.

Any idea what's happening here? Why would exim fail to open the file, but
they say "Success"? If I specify the path to the file without the .db
extension, it properly reports that the file could not be found, so that's
not what's going on...

Any ideas?