AJAX call ending in error with Status 200 / OK - why?

AJAX call ending in error with Status 200 / OK - why?

ASP.NET 4.0
jQuery 1.4

While all seems well in my request and response I get tossed into the error callback rather than the success.  I use Firebug to check the repsonse and it's:

{"d":"Hello"}



Client code:

function GetStateCertifiable() {
    var AreaID = $("#listAllAreas").val();

    $.ajax({
        url: "../WebServices/AoP.asmx/GetStateCertifiable",
        data: '{"AreaID":"' + AreaID + '"}',
        type: "POST",
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        success: function (msg) {
            alert("Success");
        },
        error: function (result) {
            alert("An error occured. \nStatus: " + result.status
                            + "\nStatus Text: " + result.statusText
                            + "\nError Result: " + result);
        },
        complete: function () {
        }
    });


Server code:

    [System.Web.Script.Services.ScriptService]
    public class AoP : System.Web.Services.WebService
    {

        [WebMethod]
        [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
        public string GetStateCertifiable(string AreaID)
        {
            return "Hello";
        }
    }

My error debugging returns the following:

Status: 200
Status Text: OK
Error Result: [object XMLHttpRequest]


I can do my thing in the Completed callback but that's a total kludge.  I can't understand why I keep ending in error with no apparent error code and a response of well formed JSON.  Any ideas?

Thanks,

BK