Getting Error On cascading Dropdownlist using javascript in MVC4

See more:

C#

using

public ActionResult compdrop(int? cid)

Error 1:There is no ViewData item of type 'IEnumerable<SelectListItem>'
and

while using

public ActionResult compdrop(int cid)

Error 2:The parameters dictionary contains a null entry for parameter 'cid' of non-nullable type 'System.Int32' for method 'System.Web.Mvc.ActionResult compdrop(Int32)' in 'singleview_Mmodel.Controllers.commonController'. An optional parameter must be a reference type, a nullable type, or be declared as an optional parameter.
Parameter name: parameters