[Vala] char variables problem

[Sepehr Aryani <sepehr aryani gmail com>]

Hi guys.
what is the problem with this code?

public void main(){
    char  ch1 = 'a';
    char? ch2 = 'b';


    stdout.printf("%c\n", ch1);
    stdout.printf("%c\n", ch2);
    stdout.printf("%s\n", ch1.to_string());
    stdout.printf("%s\n", ch2.to_string());
}

when I assign a value to ch1 and ch2, they remain empty and nothing is
printed in output.
Why.......