Sort fails

Sort fails when column heading is clicked in the following code (4.7.3 & bluemarine theme):

<?php
$rows[] = array('Cell 1', 'Cell 2', 'Cell 3', 'Cell 4');
$rows[] = array('Cell 5', 'Cell 6', 'Cell 7', 'Cell 8');
$rows[] = array('Cell 9', 'Cell 10', 'Cell 11', 'Cell 12');
$rows[] = array('Cell 13', 'Cell 14', 'Cell 15', 'Cell 16');
print theme('table', array('', array('data' => 'Last Name', 'sort' => 'asc', 'field' => '2'), 'First Name', array('data' => 'Year', 'field' => '4')), $rows);
?>