Joining table does not accept null values. mvc5 c#

Question

i have 3 tables and when the 2 table is null i have an error. what is the problem with my code.

 

var list = (from t in tracking
                        join d in _departureItemsTableService.GetAll() on t.box_id.Trim() equals d.BoxNo.Trim() into mar from submark in mar.DefaultIfEmpty()
                        join dt in _departureTableService.GetAll() on submark.DepartureId equals dt.DepartureId into dep from depart in dep.DefaultIfEmpty()

                        select new TrackingListModel
                        {
                            Id = t.Id,
                            Name = t.firstname_s + " " + t.lastname_s,
                            Departure = depart?.DepartureDate ?? String.Empty,
                        });