Booking System Help

Hi, I am currently creating a cinema system using access where a
booking can be made for a event. Each event when it is shown is
categoriesd as a performance. A booking must be made for each
performamce. I have a constructed a query between the booking, event
and peformance tables and created a subform on the booking form. The
event table is linked to the performance table which in turn is linked
to the booking table. I keep on getting the "Index or primary key
cannot contain a Null value. (Error 3058)" when attempting to bring up
the event details on the subform.
Can anyone help please?