Set class if a meta value is set within post archive

I’m attempting to add a class to posts when the meta key ‘checkbox’ is clicked. Here’s what I have:

<?PHP $checkbox = get_post_meta($post->ID, 'checkbox', true);
    if (!empty($checkbox)){ $has_video = 'icon-has_video'; }
  else {$has_video = '';}
?>
<a class='<?php echo $has_video; ?>' href='<?php the_permalink(); ?>'>

… but so far it’s not working. Any suggestions?