JQuery :: How To Pass Object In JSON That Is Not String

I'm a beginner in Jquery and I have a big doubt, follow at bellow the code to be better viewed: The method below is static and Static methods do not work with instance, For this reason I passed the 'nameLabel' that the typeis Label by parameter, however I need to catch the exception and show it to the user.

[WebMethod]
public static string RetornaNomeCliente(string idCliente,Label nomeLabel){
string nomeCliente = String.Empty;
try {
nomeCliente = Cliente.RetornaNomeCliente(idCliente);
} catch (Exception e) {
nomeLabel.Text = e.Message.ToString();
} return nomeCliente;
}

But I'm using Jquery and this method is used in a Ajax Request and I use the Json to pass the datas, how showed below:
var idCliente= $("#<%= txtIDCliente.ClientID %>").val();
var nomeLabel = $("#<%= lblMensagem.ClientID %>");
var poligonal = "{ idCliente: " + idCliente + ", + lblMensagem: " + nomeLabel + " }";
But the problem is that I need to pass in my Json the nomeLabel with Label, but always a problem happen, because the same is showed like 'indefined'. How to solve this situation?