Are structs saved in multi-thread delegate call?

[Ramon via Digitalmars-d-learn]

I have something along this way:

struct json_value
{
        ..
}
function DoDirSearch(..)
{
        immutable json_value cbk = json_value(prms.argv[3]);
        assert(cbk != json_value.init); // OK, pass
        
        import core.thread;
        new Thread({
                assert(cbk != json_value.init); // FAIL!
        }).start();
}

the problem is, my struct value is not saved in the spawned tread 
context, so this assert fails while at the start it passed:

assert(cbk != json_value.init);