subform primary key null value want custom msg

[raton]

I have a form with a subform. a primary key(not auto number) field in the subform. if i insert a number in the primary key field. then i delete the number and want to close the main form by clcking x button without saving any record. then the msg "a primary key cannot contain a null value" then a msaccess standard msg. but what i want is My custom msg and not any standard msg

[June7]

Review this http://msdn.microsoft.com/en-us/libr...ffice.11).aspx

Try:
Private Sub Form_BeforeUpdate(Cancel As Integer)
If IsNull(primary key field) Then
Me.Undo
End If
End Sub

Can put a message box in there but that might just be an annoyance for the user to have to click another button.