I found the following:
If (IsNull(Me.AMOUNTs) Or (Me.AMOUNTs = 0)) Then
If MsgBox("Did you miss AMOUNT?", vbDefaultButton1 + vbYesNo + vbQuestion) = vbYes Then
Cancel = True
'Me.AMOUNTs.SetFocus
End If
End If
working better on the OnExit event of the control.