[exim] data acl expansion question

Hi all,

I have a data acl than needs to pass the message through an external
program that will either return a value of SPAM or HAM.

condition = ${if eq{${run{/usr/local/bin/ispam $message_headers \
$message_body}}}{SPAM}{1}{0}}

however, the result seems to always be 0, regardless of the output of
the external program. Can anyone tell me what I am doing wrong?

Thanks.