使用Ajax时,服务器响应状态为500(内部服务器错误)

$.ajax({
        url: "/Orders/CheckIfExists",type: "GET",contentType: "application/json; charset=utf-8",data: {
            catalogNumber: viewModel.catalogNumber,quantity: viewModel.quantity
        },error: function (data) {
            alert("wyst?pi? nieokre?lony b??d " + data);
        },success: function (data) {
            if(data.ok)
            {
                alert(data.quantity)
            }
        }
    })
});

这是控制器方法：

public JsonResult CheckIfExists(string catalogNumber,int quantity)
    {
        List<Expression<Func<DeviceInstance,bool>>> where = new List<Expression<Func<DeviceInstance,bool>>>();
        where.Add(w=>w.DeviceUsage.UserId==1);
        where.Add(w => w.Project == null);
        where.Add(w => w.Device.CatalogNo == catalogNumber);
        var result = unitOfWork.deviceInstanceRepository.Get(where)
          .GroupBy(w => new
          {
              DeviceId = w.DeviceId,CatalogName = w.Device.CatalogNo,})
          .Select(s => new
          {
              Quantity = s.Sum(x => x.Quantity),}).First();
        if (result.Quantity >= quantity)
        {
            return Json(new { ok = true,quantity = result.Quantity});

        }
        return Json(new { ok = false });
    }

但我总是得到内部500错误.
通过方法接收数据,所有计算都可以.我在例子中编写了返回JSON.
我犯了哪个错误？