Expanding the Results View will enumerate the IEnumerable

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ASP.NET

MVC4

This is my code for update (edit) datas, which i have send object for edit data, but on this
(if (ModelState.IsValid)) statement i have getting false. and on debugging face i have getting -Result View is - "Expanding the Results View will enumerate the IEnumerable". Actually i am not understanding how can do i.

public string Edit(WrittenExam objTodo)
      {
          string msg;
          try
          {
             if (ModelState.IsValid)
              {
                  db.Entry(objTodo).State = EntityState.Modified;
                  db.SaveChanges();
                  msg = "Saved Successfully";
              }
              else
              {
                  msg = "Validation data not successfull";
              }
          }
          catch (Exception ex)
          {
              msg = "Error occured:" + ex.Message;
          }
          return msg;
      }

This is my Model

public class WrittenExam
  {
      public int id { get; set; }

      public DateTime examdate { get; set; }

      public DateTime year { get; set; }

      public string examopen { get; set; }

      public string applicationfee { get; set; }

      public string writtenfee { get; set; }


  }

Thanks.