Can't add an array element

[aben007]

string strJson = "{"ID": 3,"Weight":[{"ID": 1,"Weight": 24},{"ID": 2,"Weight": 33}]}";
var jd = JsonMapper.ToObject(strJson);
JsonData nd = new JsonData();
nd["ID"]=3;
nd["Weight"]=17;
jd["Weight"].Add(nd);
Console.Write(jd["Weight"].ToJson());
Console.Write(jd.ToJson());

The first output is right, but the second output jd.ToJson() not include nd

[aben007]

temporary plan：
...
jd["Weight"].Add(nd);
jd["Weight"]=jd["Weight"];
...

Then output is ok, but that plan is... ugly...