[exim] ACL condition not doing what I expect

Hi

I have an ACL condition that sets a variable in order to allow for
per-domain rejection messages. The ACL looks like this:

warn set acl_m99 = Not accepted.
warn set acl_m99 = ${readfile{$domain/reject-message}{}}
      condition = ${if exists{$domain/reject-message}}

The "condition=" line appears to be being ignored as, when a mail
arrives for a domain I don't deal with, I see in the logs:

failed to expand ACL string "${readfile{$domain/reject-message}{}}"


As, obviously, this file does not exist as it's not a domain I know about.

Did I miss something obvious here?

Thanks

Martin.