jsonp与ajax jQuery

I have this code:

$.ajax({
    type: "POST",
    url: URL,
    data: data,
    dataType: 'jsonp',
    crossDomain: true,
    contentType: "application/json; charset=utf-8",
    jsonpCallback: callback,
    success: function (data,textStatus,jqXHR) {
        alert("success");
    }, 
    error:function(jqXHR, textStatus, errorThrown){
        alert(JSON.stringify(jqXHR));
    },
    complete: function(jqXHR, textStatus){
        alert(JSON.stringify(jqXHR));
    }

});

the the status response is 200 but it's treated as an error. I can see this on chrome response tab:

{
  "error": "cannot create user because user already exists",
  "code": 404
}

<-- this is the data returned by the server

and I can't get that data using jQuery.

I made it to work with this code:

    $.ajax({
        type: "POST",
        url: URL,
        data: data,
        dataType: 'jsonp',
        crossDomain: true,
        contentType: "text/javascript",
        success: function (data,textStatus,jqXHR) {
             alert(JSON.stringify(data));
        }, 
        error:function(jqXHR, textStatus, errorThrown){
             alert(JSON.stringify(jqXHR));
        },
        complete: function(jqXHR, textStatus){
             alert(JSON.stringify(jqXHR));
        }

    });

hope this help.