Adding footer with add_item breaks replacement patterns

I am currently using add_item() in hook_views_pre_render() to programmatically generate a link and add it as a footer to the view:

$view->add_item('default', 'footer', 'views', 'area', array('content' => $link, 'format' => 'filtered_html'));

This code successfully adds the footer but in doing so seems to break replacement patterns that are being used elsewhere in the view. The values that are otherwise generated disappear. Commenting out the add_item() line causes them to reappear.