Getting Permission Denied errors from bup index

I get ```[Errno 13] Permission denied``` from ```bup index``` for a ```drwxr-sr-x``` directory (that I own, in a group of which I am a member); I am running my backup script from ```cron``` (on a Debian/stable system) so it does run as me, and I have no trouble accessing that directory normally.  Is ```bup``` having problems with the group-id bit being set ?