Linq：无效的Cast异常

Question

int Clinicid = Convert.ToInt32(ViewState["ClinicID"].ToString());
        string editPid = Request["editid"].ToString();

        dynamic edt = (from ed in dc.dentist_addnewpatientmodifies
                       where ed.PatientID == editPid && ed.ClinicID == Clinicid
                       select ed).FirstOrDefault();

异常详细信息：

  

System.InvalidCastException：指定的强制转换无效。