将c实体类传递给json jquery时,httpget失败

这是我的jquery:

$.ajax({
        type: 'GET',
        url: '/services/Service.asmx/FamilyHistory',
        data: JSON.stringify({
            userID: 10,
            historyID: famid
        }),
        contentType: 'application/json; charset=utf-8',
        dataType: 'json',
        success: function(val) {
            var famHist = val.d;
            alert(famHist.ID);
        },
        error: function() {
            parent.$.jGrowl('<b>Failed</b>',
                 {
                     header: 'User Action:',
                     life: 3000
                 });
        }
    });

我的班级:

public sealed class FamilyHistoryEntity
{
    public string ID { get; set; }
    public string RelativeName { get; set; }
}

我的Web服务:

    [WebMethod(EnableSession = true)]
    [ScriptMethod(ResponseFormat = ResponseFormat.Json, UseHttpGet = true)]
    public FamilyHistoryEntity FamilyHistory(int userID, string historyID)
    {
         return GetFamilyHistory(historyID, userID); // returns a FamilyHistoryEntity class
    }

问题是,我甚至不能让它在WebService上做断点,它只是抛出jQueryAjax错误事件。