Access Error 424 Object required in Access

[xAgyex]

Here is my code.

I have a table "InventoryData" with all our UPC barcodes, and want to makes sure then the user enters one of the codes on the list from the form "UPC Scan Entry Form".

But I get error 424 when the bar code is entered

Private Sub UPC_BeforeUpdate(Cancel As Integer)

If Nz(DLookup("UPC", "InventoryData", "UPC =" & UPC_Scan_Entry_Form!UPC)) Then
MsgBox ("This Defect is already listed")
End If

End Sub