This error occurs with respect to all of the cases where a combobox
represents an optional, nullable column that is constrained by a foreign key
and an attempt is made to save a new record and the user has made no manual
selection using the combobox.

The error has been narrowed down to the failure of the combobox control to
automatically bind to the assigned, initial System.DBNull.Value when the new
record is initially automatically filled in by the system. (Instead the
combobox erroneously binds to zero.) However, if the user selects the blank
item in the combobox (associated with System.DBNull.Value), then the
System.DBNull.Value does successfully bind to the control.

As a result of this binding failure, an attempt to save the record throws a
foreign-key constraint failure, and the record will not save.

Any help anyone could give would be much appreciated!