Sendmail::Milter to remove headers

[Danny Aldham]

I am trying to use the Sendmail::Milter module to remove a
Disposition-Notification-To header from email messages. The man pages
suggest that:
if ($headerf eq "Disposition-notification-to") {
print "Found $headerf to $headerv \n";
$ctx -> chgheader($headerf,0,"") || warn "Error on 0 $! \n"
;
$ctx -> chgheader($headerf,1,"") || warn "Error on 1 $! \n"
;
print "CTX is $ctx \n";
return SMFIS_CONTINUE;
}

this code should work. But I get an Error on 0 and 1, with no code returned.
I have tried setting $headerv = "" but also with no joy.
Has anyone any successful experience removing headers with this module?

Danny Aldham