Javascript matrix error

This function blows up (without an error) whenever it is called

function valid(n){   //n is a number 0-9
    if(board[Math.floor(n/3)][n%3]==0){
        return true;
    }else{
        return false;
    }
}

where board is a global 3x3 Array matrix

var board = new Array(3);
            board[0]=new Array(3);
            board[1]=new Array(3);
            board[2]=new Array(3);

The program at start fills board with all zeros. The valid(n) function breaks during the comparison and does not reach either return. I can not for the life of me figure why this doesn't work. Thanks in advance.