I think some others have noted this here, but for Javascript there is a test case of [ [true] ] which is dissallowed. Booleans are typically considered a special case of number, and behave as such in JS (i.e., 5 + true == 6). It seems that [ [ true ] ] should be true as a best practice as the trivial sudoku of 1x1 can only have one value 1 which is best represented by a boolean.