Joining table does not accept null values. mvc5 c#

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,
});

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