When I am trying to run this code in .NET framework, I get a status code 500, but I don’t know why…

This is the back end:

public ActionResult getvisit(int id)
{
    var query = __db.Visits
                    .Where(v => v.fkDoctor == id && v.fkPatient == null)
                    .Select(v => new {v.Id, v.sDate})
                    .ToList();

    List<object> visits = new List<object>();

    foreach (var item in query)
    {
        var sdatetime = new PersianDateTime(item.sDate);
        visits.Add(new {item.Id, sdatetime.Date, sdatetime.TimeOfDay});
    }

    var result = Json(visits, JsonRequestBehavior.AllowGet);

    return result;
}

Front end:

docSpan.on("change", function () {
        const id = docSpan.val();
        visitSpan.empty();
        visitSpan.append(
            "<option value=''>انتخاب کنید</option>"
        )
        $.post("/Home/getvisit", { id: id })
            .done(function (res) {
                console.log(res);
                for (let item of res) {
                    console.log(item);
                    visitSpan.append(
                        `<option value=${item.id}>${item.sdate} || ${item.stime}</option>`
                    );
                }
            })
            .fail(function () {

            })
            .always(function () {

            });
    })

I don’t have any idea why this happens.