[Exim] System filter commands

Hi,

I'm trying to set up a system filter whereby the variable $message_body is
used as the key for a file of words and if any matches are made the message
is frozen. I've got the following in my system filter file:

#Exim Filter
if "${lookup {$message_body} lsearch {/usr/exim/words}\
{SPAM}}" is SPAM then freeze
endif

I've tried sending a test message that contains a word that is listed in
/usr/exim/words, but it is being delivered, not frozen. I've read the
manual and can't see anything that is obviously incorrect. Can someone tell
me where I'm going wrong, please?

The configuration file has been set to look at the system filter.

Cheers,

Simon.