[Exim] Got error 'empty (sub)expression' from regexp

I keep getting things like this in my reject.log:

2003-11-22 17:24:32 1ANgB6-0004gD-4F H=omr-m05.mx.aol.com
[64.12.138.17] F=<> temporarily rejected after DATA: MYSQL: query
failed: Got error 'empty (sub)expression' from regexp


This is (presumably) being generated by one of these two things in my
exim.conf which involve regexp matching:

  accept  senders        = \
                   ${lookup mysql{SELECT value FROM userpref \
                                  WHERE preference='whitelist_from' \
                                  AND value rlike '${quote_mysql:${sender_address}}' } \
                                  {$sender_address}}

 deny    senders        = mysql;select sender from deny_sender where \
                                 '${quote_mysql:$sender_address}' rlike sender
          message        = Not accepted from $sender_address

Can anyone tell me what I'm doing wrong here?

The addresses it should be matching on (and, by the way, when I add
addresses in this format to my whitelists, they DO in fact seem to
work...) are of the form "*@domain.com", for the most part. Is that
correct?

Thanks,


--
Daniel Drucker / dmd@???