[exim] ldapm lookup for 'senders =' in redirect router?

Hey all,
I would like to use an ldapm lookup to restrict people from sending
to specific forwarding e-mail addresses unless they are in the correct
group. Using the senders option works fine if I put a single e-mail
address in, but when using the ldapm lookup, it fails. I know that the
lookup is working, because I am using the exact same lookup for the data
= part, and that works as expected. Here's my router config:

ldap_test:
  driver = redirect
  verify_recipient
  senders = ${lookup ldapm { \
        user="CN=A user,DC=some,DC=domain"  \
        pass="password" \
        ldap://localhost:1234/DC=some,DC=domain?\
        mail?\
        subtree?\
        (memberOf=CN=A Group,OU=groups,DC=some,DC=domain)} {$value} {}\
        }
  condition = ${if eq {$local_part} {isdept} {1} {0}}
  data = ${lookup ldapm { \
        user="CN=A user,DC=some,DC=domain"  \
        pass="password" \
        ldap://localhost:1234/DC=some,DC=domain?\
        mail?\
        subtree?\
        (memberOf=CN=A Group,OU=groups,DC=some,DC=domain)} {$value} {}\
        }

If I change senders to "senders = myemail@???", then it works
just fine. Any thoughts?

Thanks!

Jason Martens