I agree, I see no point in putting other types' values like '' or True in a Sudoku board...

Exactly what I did! Worked like a charm.

I think that at least for python the [[True]] board is supposed to be valid. After all bool is a subclass of int and [[1]] is valid as well. So imho all boards [[x]] with x==1 ==> True should also be true. Apart from my favourite kata so far

I used isinstance(bool) to pass the test, but not sure if there is any better way