theme('image') class attribute should be an array

I am using hook_preprocess_image() to add classes to the image attributes array. A PHP Fatal error occurs on print pages when attempting to add a class to the 'class' array since 'class' is a string in print.pages.inc:

<?php
  $print['logo'] = $logo_url ? theme('image', array('path' => $logo_url, 'alt' => $site_name, 'attributes' => array('class' => 'print-logo', 'id' => 'logo'))) : '';
  // Above should be:
  // $print['logo'] = $logo_url ? theme('image', array('path' => $logo_url, 'alt' => $site_name, 'attributes' => array('class' => array('print-logo'), 'id' => 'logo'))) : '';
?>