Sequence contains more than one element in mvc

Question

public ActionResult LogOn(GDCCollege.Models.StudentViewModel model, Attendance amodel)

        {

            if (ModelState.IsValid)

            {

                var v = db.Students.Where(a => a.Class.Equals(model.Class) && a.RollNo.Equals(model.RollNo)).FirstOrDefault();

                if (v == null)

                {

                    ViewBag.Message = "Invalid username and password";

                }

                else

                {

                    Session["LogedId"] = v.Id.ToString();

                    Session["LogedStudentName"] = v.StudentName.ToString();

                    var vm = db.Attendances.Single(u => u.StudentId == v.Id);

                   //var vm = (from a in db.Attendances where a.StudentId == v.Id select a).Single();

                    if (vm != null)

                    {

                        Attendance viewmodel = new Attendance();

                        viewmodel.Subject = vm.Subject;

                        viewmodel.PrevMonthAttendance = vm.PrevMonthAttendance;

                        viewmodel.Semester1Attendance = vm.Semester1Attendance;

                        viewmodel.Semester2Attendance = vm.Semester2Attendance;

                        return RedirectToAction("StudentProfile", viewmodel);

                    }

                    else

                    {

                        ViewBag.Message = "Try Again";

                    }

                }

            }

            return View(model);

        }


   There is an error occurred in this line where i highlight from green color..(sequence contain more than one element). so what do i do for it..I have multiple subjects attendence of single student.. and want o show on student profile view page.