C# threading - Lock Object

I am trying to lock a "boxed" object in a c# app, is this not possible?

    class t
    {
        System.Object t_x = new object();

        public t(int p)
        {
            t_x = p;

        }

        public void w()
        {
            lock (t_x)
            {
                for (int i = 0; i < 4; i++)
                {

                    {
                        t_x = ((int)t_x) + 1;
                        Console.WriteLine(t_x);
                        Thread.Sleep(1000);
                    }
                }
            }
        }
    }

In another class I can start 2 threads:

        Thread b1 = new Thread(new ThreadStart(t1.w));
        b1.Start();
        Thread b2 = new Thread(new ThreadStart(t1.w));
        b2.Start();

However the portion is not locked. When I lock an arbitrary object (i.e. one created and not modified as object a=new object()) it locks well. Is boxing operation somehow "depromotes" my Object??