c-sharp_null-coalescing_bug

using System;
using Newtonsoft.Json;

namespace Test {
        public class Program {
                public static void Main() {
                        string json = "{ \"phones\": { \"personal\": null }, \"birthday\": null }";
                        dynamic d = JsonConvert.DeserializeObject(json);

                        Console.WriteLine(d.phones.personal == null);

                        string s = ""; s += (d.phones.personal ?? "default");
                        Console.WriteLine(s + " " + s.Length);

                        string ss = d.phones.personal;
                        string sss = ""; sss += (ss ?? "default");
                        Console.WriteLine(sss + " " + sss.Length);

                        Console.WriteLine(d.birthday == null);

                        string s4 = ""; s4 += (d.birthday ?? "default");
                        Console.WriteLine(s4 + " " + s4.Length);

                        string s5 = d.birthday;
                        string s6 = ""; s6 += (s5 ?? "default");
                        Console.WriteLine(s6 + " " + s6.Length);

                        //works, but verbose
                        string s7 = "";
                        s7 += (d.birthday != null ? d.birthday : "default");
                        Console.WriteLine(s7 + " " + s7.Length);

                        d = null;

                        //also works
                        string s8 = d ?? "default";
                        Console.WriteLine(s8 + " " + s8.Length);
                }
        }
}