On exit Event triggering when it is not supposed to?

[Ziggy1]

I keep getting myself bogged down with what seems like it should work...

the code below is on the "on Exit" of a control on my Subform, yet it keeps triggering while I am tabbing out of fields on the main form...makes no sense, I'm not even touching the control yet?

Private Sub Skid_Exit(Cancel As Integer)


Dim x As Integer

x = Nz(Len(Me.Skid), 0)

If x = 0 Then
MsgBox " You must enter a skid or Hit Escape to exit record"
Cancel = True

End If
End Sub