[exim] lsearch error I don't understand

I'm getting this error:

2006-04-17 16:39:28 1FVdJJ-0000IC-SC <abeckman@???>: spamcheck
transport output: 2006-04-17 16:39:27 1FVdJX-0000oh-M8 Expansion of
${lookup{$2}lsearch{/etc/exim/acllists/domainaliases.txt}{$local_part@$value}{$local_part@$domain}}
failed while rewriting: lookup expansions are not permitted

It seem to happen every now and then and not sure what's triggering it.
Here's the config file:

begin rewrite

*@*
${lookup{$2}lsearch{/etc/exim/acllists/domainaliases.txt}{$local_part@$value}{$local_part@$domain}}
E

The data file here looks like this:

domain1.com: domain2.com
....

Thanks in advance.