[exim] Need ACL help - file doesn't exist problem

ok - need some quick ACL help. I'm writing the back end for this
project. Here's the situation:

deny    message = REJECTED - User listed in personal block list
        condition = 
${lookup{$sender_address}lsearch{/etc/mailcontrol/domains/perkel.com/users/marc/blockfrom.txt}{yes}{no}}

The above code works of the file exists but if the file doesn't exist
then I have problems. What I need is for it to return {no} if the file
isn't there. How do I do that? Thanks in advance.

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Marc Perkel - marc@???

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