linq group have a question?

[界]

first! my english so poor...

please excuse me= ="

i need select  a( List<DeputyLeaveItem> source) and group by it

but it sometimes OK~sometimes error:object reference not set to an instance of an object

this is my code:

var resultList = (from a in source.ToList()
                          group a by new { a.leaveStartDate, a.leaveEndDate }
                              into b
                              select new LeaveItem()
                              {
                                  showOn = DateTime.Now.Date <= Convert.ToDateTime(b.First().leaveEndDate).Date ? "Y" : "N",
                                  delFlag = DateTime.Now.Date <= Convert.ToDateTime(b.First().leaveEndDate).Date,
                                  leaveStartDate = b.First().leaveStartDate,
                                  leaveEndDate = b.First().leaveEndDate
                              }).OrderByDescending(x => x.leaveStartDate).ToList();