Do custom fields work inside a WP_Query loop inside a page?

I’ve got a page which uses this to insert a series of items into the body of the content (using exec-php plugin):

<ul>
<?php $my_query = new WP_Query('cat=XX');
  while ($my_query->have_posts()) : $my_query->the_post(); ?>
	<li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
<?php endwhile; ?>
</ul>

That works fine except that now I want to replace the_permalink with a URL entered into a custom field, and if I do this:

<ul>
<?php $my_query = new WP_Query('cat=XX');
  while ($my_query->have_posts()) : $my_query->the_post();
	$link = get_post_meta($post->ID, 'site-url', true); ?>
	<li><a href="http://<?php echo $link; ?>"><?php the_title(); ?></a></li>
<?php endwhile; ?>
</ul>

the link comes out blank.

So, is it not possible to use custom fields in this manner, ie, inside a custom loop inside a normal WP loop?