[exim] Mysq llookup fails in exiscan acl

Can someone tell me why the mysql lookup fails in acl_smtp_data?

acl_exiscan:
  warn  message = X-Cheetah-Spam: True
          condition = ${if ! def:h_X-Spam:}
          condition = ${if <{$message_size}{200k}{1}{0}}
          set acl_m4 =  ${lookup mysql{DELIVER}{$value}{0}}
          spam = mail/defer_ok

This same lookup succeeds in routers and other acls, but not in this
one. Always returns 0. (using 4.50)

Thanks,
Craig