issue - Pear's Spreadsheet_Excel_Writer creates a 0 byte file

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When I run the code below, it create a file but it is 0 bytes and throws an  
error when I try to open it, "Unknown or unsupported file format."

Here's the test code:
require_once 'Spreadsheet/Excel/Writer.php';
$workbook = new Spreadsheet_Excel_Writer();
$worksheet =& $workbook->addWorksheet('My first worksheet');
$worksheet->write(0, 0, 'Name');
$worksheet->write(0, 1, 'Age');
$worksheet->write(1, 0, 'John Smith');
$worksheet->write(1, 1, 30);
$worksheet->write(2, 0, 'Johann Schmidt');
$worksheet->write(2, 1, 31);
$worksheet->write(3, 0, 'Juan Herrera');
$worksheet->write(3, 1, 32);

My installation is:
PHP 4.3
OpenOffice 2.3
OLE                                  0.5     beta
Spreadsheet_Excel_Writer             0.9.1   beta

Any ideas what's going wrong?  

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