I'm trying to get the SUM of "bookings" and I get error "The cast to value type 'Int32' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type."

var bookings = entities.Bookings.Where(x => x.ID == id &&
                                     x.StartDate <= bookingEnd &&
                                     x.EndDate >= bookingStart)
                                    .Sum(x => x.BookingQuantity);

How shall I fix this? I need to get 0 if it ever gets to be null else its bookings.

linq entity-framework
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