[exim] Problem with prvs functions

I'm just trying out the prvs facility added in 4.52, but I can't get it to
validate a signature properly.

# exim -be '${prvs {brian@???}{foobar}{1}}'
prvs=brian/19844dfc9a@???

# exim -be '${prvscheck {prvs=brian/19844dfc9a@???}{foobar}{$prvscheck_result}}'
brian@???

OK so far - but if I break either the signature or the secret, I still get a
validated answer:

# exim -be '${prvscheck {prvs=brian/0123456789@???}{foobar}{$prvscheck_result}}'
brian@???

# exim -be '${prvscheck {prvs=brian/19844dfc9a@???}{wibble}{$prvscheck_result}}'
brian@???

Actually, according to doc/NewStuff, $prvscheck_result should either be
empty string or "1", not an E-mail address.

Am I doing something wrong, or is the doc broken, or is the code broken?

Thanks,

Brian.

# exim -bV
Exim version 4.52 #1 built 13-Jul-2005 14:47:33
Copyright (c) University of Cambridge 2005
Berkeley DB: Sleepycat Software: Berkeley DB 4.3.28: (April 22, 2005)
Support for: Expand_dlfunc OpenSSL Content_Scanning
Lookups: lsearch wildlsearch nwildlsearch iplsearch cdb dbm dbmnz ldap ldapdn ldapm
Authenticators: cram_md5 plaintext
Routers: accept dnslookup ipliteral manualroute queryprogram redirect
Transports: appendfile/maildir autoreply pipe smtp
Fixed never_users: 0
Configuration file is /usr/linnet/0.HEAD/conf/configure
#