Spreadsheet_Excel_Writer: How to merge 2 cells?

Hi ,
I am trying Spreadsheet_Excel_Writer for the first time. My requirement involves the formatting of cells. Here am trying to merge columns as well as the rows. But the row merge doesnt work.

Code:
<?php
require_once 'Spreadsheet/Excel/Writer.php';
$workbook = new Spreadsheet_Excel_Writer();

$format_bold =& $workbook->addFormat();
$format_bold->setBold();

//merge the rows
$format_merge_rows =& $workbook->addFormat();
$format_merge_rows->setMerge(4,0,5,0);

$format_title =& $workbook->addFormat();
$format_title->setBold();
$format_title->setColor('yellow');
$format_title->setPattern(1);
$format_title->setFgColor('blue');
// let's merge
$format_title->setAlign('merge');

$worksheet =& $workbook->addWorksheet();
$worksheet->write(0, 0, "Quarterly Profits for Dotcom.Com", $format_title);
// Couple of empty cells to make it look better
$worksheet->write(0, 1, "", $format_title);
$worksheet->write(0, 2, "", $format_title);
$worksheet->write(1, 0, "Quarter", $format_bold);
$worksheet->write(1, 1, "Profit", $format_bold);
$worksheet->write(2, 0, "Q1");
$worksheet->write(2, 1, 0);
$worksheet->write(3, 0, "Q2");
$worksheet->write(3, 1, 0);
$worksheet->writeString(4, 0, "Q3", $format_merge_rows);
$worksheet->write(5, 0, "", $format_merge_rows);
$worksheet->write(4, 1, 0);
$worksheet->write(5, 1, 10);


/* Writing the worksheet according to out requirement */

$workbook->send('test.xls');
$workbook->close();
?>

In row 0, the columns 0,1,2 comes to be merged. But the cells (4,0) and (5,0) does not merge. Please help.

Thank you
Faizal