[Vala] string parameter in static function

[lariamat <interflug1 gmx net>]

I get a strange c compilation warning when using a string as parameter
in a static function. When i forward the string to another string
everything is OK. 
(In my program I get this warning from a Gst.TagForeachFunc.)
I am using vala 0.5.2 but I also had this warning in previous versions.
Here are two testcases. The first is giving the warning, the second not.

FIRST:
------------------------------------------
public static void function_1(int i, string a_string) {
    switch (a_string) {
        case "lalala_1":
            print("lalala_1\n");
            break;
        default:
            print("default case\n");
            break;
        }
}
public static int main (string[] args) {
    function_1(1, "lalala_1");
    return 0;
}
------------------------------------------

SECOND:
------------------------------------------
public static void function_1(int i, string a_string) {
    string x = a_string;
    switch (x) {
         case "lalala_1":
              print("lalala_1\n");
              break;
         default:
              print("default case\n");
              break;
    }
}
public static int main (string[] args) {
    function_1(1, "lalala_1");
    return 0;
}
------------------------------------------

Is this a bug or am I getting something wrong?
Regards