Totalling a column of fields gives GPF ?

Can anyone please help.

I have a DBGrid displaying 7 columns of fields. The last column is a
calculated field (LineTotal) the result of column 2 multiplied by 4 (Qty
x Unit cost).

I am trying to total up the Line Totals to give a Grand Total using the
following code. Everytime I do I get a GPF. Any ideas please.

procedure TFrm_Invoice.TableInvoiceBodyCalcFields(DataSet: TDataset);
var
  TempTotal: Extended;
  PrevRecord: TBookmark;
begin
TableInvoiceBodyLine_Ttl.value := TableInvoiceBodyQty.value *
                  TableInvoiceBodyUnit_pr.value;
PrevRecord := TableInvoiceBody.GetBookmark;
  try
    TableInvoiceBody.DisableControls;
    TableInvoiceBody.First;
    TempTotal := 0;
    while not TableInvoiceBody.EOF do
    begin
      TempTotal := TempTotal + TableInvoiceBodyLine_Ttl.Value;
      TableInvoiceBody.Next;
    end;
    TableInvoiceBodyOurTotal.Value := TempTotal;
  finally
     TableInvoiceBody.EnableControls;
     if PrevRecord <> nil then
     begin
       TableInvoiceBody.GoToBookmark(PrevRecord);
       TableInvoiceBody.FreeBookmark(PrevRecord);
     end;
  end;
end;

Regards, Ian
                FOREST LANDROVERS' 4 x 4 CENTRE
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