[exim] error "failed to expand "${lookup ldap..." in require…

I'm trying to set up some require_files on a router I have. I've tried the
following require_files lines:

require_files = ${lookup ldap {ldap:///ou=<ou>,o=<o>?homeDirectory?sub?(uid=${quote_ldap:$local_part})}{$value}{}}/file:/path/to/second/file
require_files = ${lookup ldap {ldap:///ou=<ou>,o=<o>?homeDirectory?sub?(uid=$local_part)}{$value}{}}/file:/path/to/second/file
require_files = ${lookup ldap {ldap:///ou=<ou>,o=<o>?homeDirectory?sub?(uid=${quote_ldap:$local_part})}}/file:/path/to/second/file
require_files = ${lookup ldap {ldap:///ou=<ou>,o=<o>?homeDirectory?sub?(uid=$local_part)}}/file:/path/to/second/file

Each of them gives me the following error when I try to verify an address
that would use the router:

failed to expand "${lookup ldap {ldap" for require_files: missing } at end
of string

In one of my transports, I have a similar line that works:

directory = ${lookup ldap {ldap:///ou=<ou>,o=<o>?homeDirectory?sub?(uid=$local_part)}}/Maildir

What's wrong with the require_files lines?

--
Mark T. Valites
Unix Systems Analyst
Computing & Information Technology
SUNY Geneseo
>--))> >--))>