Spreadsheet_Excel_Writer: Best way to write '#N/A'

I would like to know the best way to write the missing or null value '#N/A' using Spreadsheet_Excel_Writer. The best way I've found is: $worksheet->writeFormula($row, $col, "=NA()"); This imports correctly into in Excel, OpenOffice, and Gnumeric but is converted to 0 in KSpread. Spreadsheet_Excel_Writer stores the formula '=NA()' and the value 0 in a cell and sets a variable that tells the spreadsheet software to reevaluate all formulas when the spreadsheet is opened. This is supposed to replace the value 0 with '#N/A'. I don't think the obvious works. $worksheet->write($row, $col, null); This creates an empty cell. $worksheet->write($row, $col, '#N/A'); This writes the string '#N/A' which is not a special value. I would appreciate any other ideas. Thanks, George Essig