sb_filter.py eats mail when used in postfix content_filter

OS:  CentOS 6.4 x86_64
Postfix package:  postfix-2.6.6-2.2.el6_1.x86_64
Spambayes package:  spambayes-1.1-0.2.a6.el6.noarch

All right, I've hit a brick wall with my integration of spambayes and 
postfix.  I based my build combining Jonathan St. Andre's instructions 
at http://spambayes.sourceforge.net/server_side.html, with a few tweaks 
from the Postfix side in http://www.postfix.org/FILTER_README.html --- 
mostly because I'm going to extend the logic.

I've successfully trained spambayes with about 12,000 each spam and ham, 
creating a database in /var/spambayes/filter.db.

Running a captured message through sb_filter.py from the command line 
works flawlessly:
-----------------------------------------
$ whoami
filter
$ cat /tmp/msg.7440 | /usr/bin/sb_filter.py -d /var/spambayes/filter.db -f
Return-Path: <[email protected]>
Received: from amhran.net (3wg23q1.amhran.net [192.168.1.160])
         by courier.amhran.net (Postfix) with ESMTP id CFF223E
         for <[email protected]>; Wed, 24 Apr 2013 21:28:34 -0400 (EDT)
From: "EMF" <[email protected]>
To: "EMF" <[email protected]>
Subject: Testing
X-Spambayes-Classification: ham; 0.17

SB Test
$
-----------------------------------------

Here are the lines from postfix's master.cf (which are very 
straightforward):
-----------------------------------------
smtp      inet  n       -       n       -       -       smtpd
      -o content_filter=filter:dummy
filter unix  -   n   n   -   -   pipe
      flags=Rq user=filter
      argv=/usr/local/bin/spamfilter.sh -f ${sender} -- ${recipient}
-----------------------------------------

Spamfilter.sh sends the mail just fine if it consists of this:
-----------------------------------------
     cat > /tmp/msg.$$
     /usr/sbin/sendmail -i $@ < /tmp/msg.$$
-----------------------------------------
but not if it contains this:
-----------------------------------------
     cat > /tmp/msg.$$
     cat /tmp/msg.$$ | /usr/bin/sb_filter.py -d /var/spambayes/filter.db 
-f > /tmp/new.$$
     /usr/sbin/sendmail -i $@ < /tmp/new.$$
-----------------------------------------

In the latter case --- msg.$$ contains the message, but new.$$ is a 
zero-length file, and thus what comes through on the far end is an empty 
message.  Interestingly, I get this in syslog.log:
-----------------------------------------
Apr 24 21:28:35 courier abrt: detected unhandled Python exception in 
'/usr/bin/sb_filter.py'
Apr 24 21:28:35 courier abrt: can't communicate with ABRT daemon, is it 
running? [Errno 13] Permission denied
-----------------------------------------

How can I capture the exception?  What's being called by sb_filter.py at 
the command line that isn't available when run inside the content_filter?
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