First of all, sorry!
This is intended because its not clear how many spaces are in a tab, which is a problem when you mix tabs and spaces together (I think I recall python not being able to do this very well, but it's been a while.) So I went for strict spaces only indentation. As it happens the vim and textmate/sublime modes make tabs big and red as a warning. Unfortunately the parser gives no hint so I'm very sorry if you spent a lot of time wondering what was going on!
Feel free to add an issue on GitHub for a better error message though.