[Exim] lookup question

I'm attempting to install mailman 2.1.3 with exim 4.24 and having some issues with the transport. I want to use virtual servers, so I need to lookup the proper path to the correct mailman command based on the domain.

${lookup pgsql{select homedir from domains where domain='$domain' limit 1} {$value} fail}/mailman/mail/mailman

fails by telling me that there is a missing lookup type. If I cut'n'paste a random pgsql lookup from elsewhere in the config where that works, it still cannot look up in this transport. Is there something unusual about 'command' that makes it different from the other lookups?

Kelley