Dear Bob,
Thanks again for your quick reply.
I adjusted your code so now it looks like this:
Private Sub txtMedewerker_ID_LostFocus()
If Me.txtMedewerker_ID = "" Or IsNull(Me.txtMedewerker_ID) Then
Exit Sub
Else
If IsNumeric(Me.txtMedewerker_ID) And InStr(1, ",", Me.txtMedewerker_ID...