Использование HTML-формы

Я пытаюсь добавить на свою страницу поисковую форму:

@using (Html.BeginForm("CheckCourses", "Home", new { programId = ViewBag.ProgramId }, FormMethod.Get))
{
    <p class="col-md-12">
        Find by course or teacher name: @Html.TextBox("SearchString", ViewBag.CurrentFilter as string)
        <input type="submit" value="Search Course" class="btn-primary" />
    </p>
}

Теперь CheckCourses() должен иметь programId для запуска (не допускающий значения NULL). Вот почему я добавил new { programId = ViewBag.ProgramId } в routeHelper. ViewBag.ProgramId не является нулем во время выполнения этого кода, однако, когда я нажимаю кнопку Search Course, я получаю сообщение о том, что значение programId равно нулю: The parameters dictionary contains a null entry for parameter 'programId' of non-nullable type 'System.Int32' for method 'System.Web.Mvc.ActionResult CheckCourses(Int32, System.String, System.Nullable 1[System.Int32])' in 'TanulmanyiRendszer.Controllers.HomeController'. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter. Parameter name: parameters

Полная трассировка стека:

[ArgumentException: The parameters dictionary contains a null entry for parameter 'programId' of non-nullable type 'System.Int32' for method 'System.Web.Mvc.ActionResult CheckCourses(Int32, System.String, System.Nullable`1[System.Int32])' in 'TanulmanyiRendszer.Controllers.HomeController'. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter.
Parameter name: parameters]
   System.Web.Mvc.ActionDescriptor.ExtractParameterFromDictionary(ParameterInfo parameterInfo, IDictionary`2 parameters, MethodInfo methodInfo) +527
   System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +91
   System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +27
   System.Web.Mvc.Async.AsyncControllerActionInvoker.<BeginInvokeSynchronousActionMethod>b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState) +22
   System.Web.Mvc.Async.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult) +29
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +32
   System.Web.Mvc.Async.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d() +50
   System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +225
   System.Web.Mvc.Async.<>c__DisplayClass33.<BeginInvokeActionMethodWithFilters>b__32(IAsyncResult asyncResult) +10
   System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +10
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult) +34
   System.Web.Mvc.Async.<>c__DisplayClass2b.<BeginInvokeAction>b__1c() +26
   System.Web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeAction>b__1e(IAsyncResult asyncResult) +100
   System.Web.Mvc.Async.WrappedAsyncResult`1.CallEndDelegate(IAsyncResult asyncResult) +10
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
   System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +27
   System.Web.Mvc.Controller.<BeginExecuteCore>b__1d(IAsyncResult asyncResult, ExecuteCoreState innerState) +13
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +29
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
   System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +36
   System.Web.Mvc.Controller.<BeginExecute>b__15(IAsyncResult asyncResult, Controller controller) +12
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +22
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
   System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +26
   System.Web.Mvc.Controller.System.Web.Mvc.Async.IAsyncController.EndExecute(IAsyncResult asyncResult) +10
   System.Web.Mvc.MvcHandler.<BeginProcessRequest>b__5(IAsyncResult asyncResult, ProcessRequestState innerState) +21
   System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +29
   System.Web.Mvc.Async.WrappedAsyncResultBase`1.End() +49
   System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +28
   System.Web.Mvc.MvcHandler.System.Web.IHttpAsyncHandler.EndProcessRequest(IAsyncResult result) +9
   System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +9765045
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +155

ОБНОВЛЕНИЕ

Контроллер по запросу:

public class HomeController : BaseController
{
    public ActionResult Index()
    {
        return View("Index", _programService.Programs.ToList());
    }

    public ActionResult CheckCourses(int programId, string searchString, int? page) {
        var courses = from s in _courseService.Courses select s;
        if (searchString != null)
        {
            page = 1;
        }

        ViewBag.CurrentFilter = searchString;
        ViewBag.ProgramId = programId;

        if (!String.IsNullOrEmpty(searchString))
        {
            searchString = searchString.ToLower();
            courses = courses.Where(s => s.CrsName.ToLower().Contains(searchString) || s.EducationUser.FullName.ToLower().Contains(searchString));
        }
        courses = courses.OrderBy(s => s.Id);
        int pageSize = 20;
        int pageNumber = (page ?? 1);

        return View("Courses",courses.ToPagedList(pageNumber, pageSize));
    }

}

ОБНОВЛЕНИЕ 2

Визуализированный элемент <form> показывает, что функция работала хорошо, однако проблема все еще сохраняется при нажатии кнопки поиска:

<form action="/Home/CheckCourses?programId=2" method="get">            
    <p class="col-md-12">
        Find by course or teacher name: <input id="SearchString" name="SearchString" type="text" value="">
    <input type="submit" value="Search Course" class="btn-primary">
    </p>
</form>