[AccessD] Prevent Duplicates

I need a way to check if a vendor number is a duplicate entry. On the
AfterUpdate of VenNumber I placed the below code, but it produces an error
about not finding the Automation Object and gives the entry I made in the
VenNumber field. VenNumber is a text field - do I have the quotes or
parenthesis wrong?

Virginia

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 If Not IsNull(VenNumber) Then
    If DCount("VenNumber", "tbl_Vendor", "VenNumber = " & [VenNumber]) > 0
Then   
        MsgBox "You have entered a Vendor Number that already exists"
        VenNumber.SetFocus
        VenNumber.Undo
End If
    End If