My guess is that not many people use this option... I have just tested it and confirmed this bug. See the issue I just lodged on the TKL Issue Tracker.

A workaround may be to use the live part of the TKL ISO/CD/USB to launch fdisk and manually configure your partitions then re-run the TKL installer (although I haven't tested that and perhaps you may run into the same issue).

Even if that doesn't work, you may be able to manually install from the commandline of the live system. I have recently done this myself and it worked quite well. I'll try to dig up my notes and post back (please feel free to post back and give me a nudge if you don't hear back soon...)