"Invalid JSON primitive" in Ajax processing

I am getting an error in an ajax call from jQuery.

Here is my jQuery function:

function DeleteItem(RecordId, UId, XmlName, ItemType, UserProfileId) {
    var obj = {
        RecordId: RecordId,
        UserId: UId,
        UserProfileId: UserProfileId,
        ItemType: ItemType,
        FileName: XmlName
    };
    var json = Sys.Serialization.JavaScriptSerializer.serialize(obj);

    $.ajax({
        type: "POST",
        url: "EditUserProfile.aspx/DeleteRecord",
        data: json,
        contentType: "application/json; charset=utf-8",
        dataType: "json",
        async: true,
        cache: false,
        success: function(msg) {
            if (msg.d != null) {
                RefreshData(ItemType, msg.d);
            }
        },
        error: function(XMLHttpRequest, textStatus, errorThrown) {
            alert("error occured during deleting");
        }
    });
}

and this is my WebMethod:

[WebMethod]
public static string DeleteRecord(Int64 RecordId, Int64 UserId, Int64 UserProfileId, string ItemType, string FileName) {
    try {
        string FilePath = HttpContext.Current.Server.MapPath(FileName);

        XDocument xmldoc = XDocument.Load(FilePath);
        XElement Xelm = xmldoc.Element("UserProfile");
        XElement parentElement = Xelm.XPathSelectElement(ItemType + "/Fields");

        (from BO in parentElement.Descendants("Record")
         where BO.Element("Id").Attribute("value").Value == RecordId.ToString()
         select BO).Remove();
        XDocument xdoc = XDocument.Parse(Xelm.ToString(), LoadOptions.PreserveWhitespace);
        xdoc.Save(FilePath);

        UserInfoHandler obj = new UserInfoHandler();
        return obj.GetHTML(UserId, UserProfileId, FileName, ItemType, RecordId, Xelm).ToString();
    } catch (Exception ex) {
        HandleException.LogError(ex, "EditUserProfile.aspx", "DeleteRecord");
    }
    return "succ