Håndtering af NULL-værdier ved beregning

Ved afvikling af følgende kode får jeg:

Microsoft VBScript runtime error '800a000d'
Type mismatch

/adm_pp.asp, line 42

line 42 er:
disc = (brutto - netto)

det skyldes formentlig, at rs2("brutto") er en NULL-værdi. Jeg har forsøgt med FormatNumber men uden held.
'uddrag fra koden.....
'---------------------
strSQL = "SELECT * FROM product WHERE package = True ORDER BY prodarea_id, prod_sort"
Set rs = Conn.Execute(strSQL)

if Not rs.EOF then
Response.Write t
Do Until rs.EOF
  brutto = 0
  netto = FormatNumber(rs("price"),2)
  disc = 0
  strSQL2 = "SELECT SUM(price) AS brutto FROM product WHERE product_id IN (SELECT sub_product_id FROM package WHERE main_product_id = " & rs("product_id") & ")"
  Set rs2 = Conn.Execute(strSQL2)
  if Not rs2.EOF then
  brutto = FormatNumber(rs2("brutto"),2)
  end if
  disc = (brutto - netto)
  rs2.Close
  i = i + 1
  if i mod 2 = 0 then
  Response.Write "<tr id='bgcolor2'>"
  else
  Response.Write "<tr>"
  end if
  Response.Write "<td>" & rs("prodtext_short") & "</td>"
  Response.Write "<td align='right'>" & brutto & "</td>"
  Response.Write "<td align='right'>" & netto & "</td>"
  Response.Write "<td align='right'>" & discount & "</td>"
  Response.Write "<td align='center'><a href='adm_pp.asp?A=DETAIL&id=" & rs("product_id") & "'><img src='images/pil.gif'></a>"
  Response.Write "</tr>"
rs.MoveNext
Loop
Response.Write "</table>"
end if