Xmal designer won't load when trying to create windows 10 universal app

I've started working on Windows 10 IoT apps, and I'm baffled by the following issue:

The XAML designer won't load when trying to create Windows 10 universal apps in Windows 8 VS 2015 RC. Upon comparing the blank project XAML for Windows 8 applications to the new blank project XAMLfor windows, they appear to be identical.

Except when I double click my Windows 10 Mainpage.xmal I only see the code and the show designer button doesn't do anything.