Display Message When Lookup Returns #N/A

I'm trying to generate a message box when a lookup cell returns the #N/A error value.

The lookup formula (=VLOOKUP(O8,Prod_Lookup,6,FALSE)) is in a "helper column" and returns a value of 0 if there is no entry in the lookup source sheet, but returns a value of #N/A if a non-matching date is entered in the active worksheet.

I have the following, but need some help in getting it work correctly. At present, it doesn't trigger the message box to pop up.

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

Dim Prod_Val As Range
Dim note1 As String

Set Prod_Val = Worksheets("C_Data").Range("P8:P7000")

note1 = "Brazer PPM cannot be calculated because the Daily Production Count" & vbLf & _
"has not been entered for this date." & vbLf & vbLf & _
"The Daily Production Count must be entered prior to entering the daily leak data." & vbLf & vbLf & _
"Upon clicking the OK button you will taken to the Production Count sheet to enter the data."