[Plugin: Related Links] Problem displaying links of type 'custom'

I have some links that are of type ‘custom’ (i.e. links to other, external websites). I want to give these a different icon, so I’m trying to list them separately from post and page links. In order to list them separately, I am using:

<?php $related_links = get_related_links("custom"); ?>
<?php foreach ($related_links as $link): ?>
<li><a class="external" href="<?php echo $link["url"]; ?>"><?php echo $link["title"]; ?></a></li>
 <?php endforeach; ?>

That is fine – it shows just the custom (external) links. However, when I try to get links of type ‘post’, it is bringing in the custom ones too:

<?php $related_links = get_related_links("post"); ?>
            <?php foreach ($related_links as $link): ?>
                <li><a class="internal" href="<?php echo $link["url"]; ?>"><?php echo $link["title"]; ?></a></li>
            <?php endforeach; ?>

https://www.ads-software.com/extend/plugins/related-links/