Drupal tabledrag broken?

<?php
$output = '';
$output .= "This works:";
$header = array('First name', 'Last name', 'Weight');
$rows = array();
$rows[] = array(
  'data' =>array(
    array('data' => 'Mark'),
    array('data' => 'Smith'),
    array('data' => '<div class="test-weight form-item">2</div>'),
  ),
  'class' => array('draggable'),
);
drupal_add_tabledrag('test-table', 'order', 'sibling', 'test-weight');
$output .= theme('table', array('header' => $header, 'rows' => $rows, 'attributes' => array('id' => 'test-table')));

$output .= "This is broken:";
$header = array('First name', 'Last name', 'Weight');
$rows = array();
$rows[] = array(
  'data' =>array(
    array('data' => 'Mark Smith', 'colspan' => 2),
    array('data' => '<div class="test-weight form-item">2</div>'),
  ),
  'class' => array('draggable'),
);
drupal_add_tabledrag('test-table', 'order', 'sibling', 'test-weight');
$output .= theme('table', array('header' => $header, 'rows' => $rows, 'attributes' => array('id' => 'test-table')));
return $output;
?>