Error when importing OLP Song Database

Getting an error when trying to import an OLP Song Database.

The traceback error is in the attached text file.

It's a song database created on a Mac (El Capitan, latest OLP release) and attempting - primarily - to import it onto a Windows 8.1, also latest OLP release.

What I've tried:
 - Importing the Mac DB onto the Windows install;
 - Importing the Windows DB onto the Mac install;
 - Both OLP installations are the latest download available on the main OLP website for the specified platform;

The attached traceback comes from the following attempt:
 - Moving the Windows DB to elsewhere on the computer,
 - Copying the Mac DB into the Windows data folder,
 - - This worked; the Mac DB is fully usable on the Windows machine; however, attempts to import the Windows DB back into OLP caused the same error and resulted in the attached traceback.

I hope that made some sort of sense there!

As a guess judging by Line 5 of the attached file, I'm wondering if I've come across another bug to do with the 'multiple songbook' feature!