Problems with crypto scanning done header

I'm trying to add the cryptographic scanning done header directly from the
exiscan-acl-examples.txt document, and any time I try to send mail through
the server I get a temporarily unavailable (451) error.

Per the instructions, I added both sections to my acl_smtp_data ACL and
they are as follows (each all on one line, and xxx = the sring I'm using
for the secret):

  accept condition = ${if eq
{${hmac{md5}}{xxx}{$body_linecount}}}{$h_X-Scan_Signature:} {1}{0}}

and at the very end, before the accept condition:

warn message = X-Scan-Signature: ${hmac{md5}{xxx}{$body_linecount}}

If I comment them out and restart Exim, everything works fine. If I
re-enable them and restart Exim, I once again get the error message.

Am I missing something? I'm using Exim 4.30 on FreeBSD 4.9 from the
FreeBSD Ports tree (which includes Exiscan, though I'm not certain which
version).

Thanks,
Mike Oliveri
Systems Administrator
UTI Systems, Inc.