Weird errors with gtar on 5.3-BETA7

I've been having problems with my nightly Amanda backups for a few weeks; some 
of the filesystems on my FreeBSD server aren't being tarred.  I've traced it 
back to an error condition in GNU tar that I don't really understand:

  # gtar clf /dev/null /
  gtar: Removing leading `/' from member names
  gtar: /proc: Cannot savedir: Invalid argument
  gtar: Error exit delayed from previous errors

I'm not sure what the "Cannot savedir: Invalid argument" message means and 
Google wasn't much help.  Any thoughts?
-- 
Kirk Strauser