VBA Code To Display A Msg Box If The Returned Value Is #N/D

I have a sheet where I put a code from a product in Column B, and the name of the product will appear in Column C, with a VLOOKUP Formula. What I would like to do is, get a VBA code to display a msg box, if somebody enter a the wrong code. If that happen a #N/D will appear in Column C.
In this Sheet I already have the bellow VBA Code, to avoid repeated product codes:

Private Sub Worksheet_Change(ByVal Target As Range)
Dim Rng As Range, Dn As Range
If Target.Count 1 Then Exit Sub
Set Rng = Range(Range("B1"), Range("A" & Rows.Count).End(xlUp))
If Application.CountIf(Rng, Target) > 1 Then
MsgBox "O valor introduzido " & "(" & Target & ")" & " é duplicado. O Menino está a Dormir???"
End If
End Sub