[Vala] error: Non-constant field initializers, but the initializer seems constant

[Nicolas CARRIER <nicolas carrier parrot com>]

Hello,
I have this kind of pattern in my code:

namespace Plop {
        int my_int = -1;
        static void check_my_int_is_initialized() {
                if (my_int < 0)
                        my_int = 42;
        }
        public static int main(string[] args) {
                check_my_int_is_initialized();
                stdout.printf(@"my_int is $my_int\n");
                return 0;
        }
}

and when I try to compile it, I get the following error:
$ valac main.vala
main.vala:2.2-2.11: error: Non-constant field initializers not supported in this 
context
        int my_int = -1;
        ^^^^^^^^^^
Compilation failed: 1 error(s), 0 warning(s)

I don't understand the issue, because I would've though that -1 is a constant 
value. Isn't it the case for vala ? Or is it an inadequate error message for 
another error which I am missing ?

Thank you in advance !