How generate a blank envelope-sender

I've been reading through the documentation, but I can't find the 
answer.  Can someone tell me how to generate a blank envelope-sender in 
an outgoing message.  I want to generate a message that won't bounce if 
it is not deliverable.
The following program generates the following error:

Can't call method "isa" on an undefined value at 
/usr/lib/perl5/site_perl/5.8.1/Mail/Transport/SMTP.pm line 48.

#!/usr/bin/perl -w
use Mail::Message;
use Mail::Transport::SMTP;
use Mail::Message::Field;
use Mail::Address;
use strict;

my $message = Mail::Message->build (
        From       => Mail::Address->new('MD',
                'matthew@xxxxxxxxxxxxxxxxxx'),
        Sender     => '<>',
        To         => 'matthew@xxxxxxxxxxxxxxxxxx',
        Subject    => 'test',
        data       => 'foo'
);

my $mailer = Mail::Transport::SMTP->new('hostname' => 'ottonexc1');
if ($mailer->send($message)) {
        print "ok\n";
} else {
        print "oops\n";
}