Runtime Error '-2147352567 On BeforeUpdate Event

I've got a form with Text boxes StartDate (datatype = Date / Time) and Interval (datatype = Numeric).
Both text boxes are bound controls.

I'd like to validate whether summing the StartDate with the Interval entered results in a calculated date, falling on either Saturday or Sunday.
If so, the Interval needs to be reset so that the calculated day will fall on the first Monday falling after the StartDate.

The code I've got so far results in an error:

Run-time error '-2147352567 (80020009):

The macro or function set to the BeforeUpdate or ValidationRule property for this field is preventing Microsoft Access from saving the data in the field.

I can understand why this error is generated.
What I can't figure out is how to adapt the code.
The AfterUpdate event is no option as this is triggered too late...

BTW, I'm on Access2000

Here's the code I've got so far:

Private Sub Interval_BeforeUpdate(Cancel As Integer)

Dim Response
Dim strInterval As Integer

If Weekday(Me.Startdatum + Me.Interval, vbMonday) = 6 Or Weekday(Me.Startdatum + Me.Interval, vbMonday) = 7 Then
Response = MsgBox("Eerstvolgende klusdatum valt in het weekend, laten vallen op maandag?", vbYesNo)
End If
If Response = vbYes Then
strInterval = 8 - Weekday(Me.Startdatum, vbMonday)
Me.Interval = strInterval
Else
If Response = vbNo Then
Me.Undo
End If
End If

End Sub

Regards,

RV