Tellico doesn't seem to handle ISBN-13

I'm going to try to kill two questions with one post here.

The core problem is that while Tellico accepts ISBN-13 numbers, searches on these fail to produce any results. Does anybody have a solution for this?

The ancillary question is about how to debug external source scripts. I've written a shell script to do an Amazon ISBN-13 look-up which works from the command line, and created a source from it in Tellico... however, the "Update entry" command returns immediately and does not fill in the fields. It could be that the script isn't being executed, or there's some other failure... but how can I tell? Is it possible to turn on debugging, or is there a Tellico log somewhere that I can look at?

Any pointers would be greatly appreciated. I'll also ask on the Tellico mailing list, and post any reply here.

Thank you.