Custom RSS with the_excerpt

I have a custom RSS feed with the following code snippet:

<description><?php echo '<![CDATA['.'<p><span>by </span><strong>'.get_post_meta($post->ID,'author',true).'</strong></p><br>'.the_excerpt_rss($post->ID).'<a href="'.get_permalink($post->ID).'">Read the rest of this story.</a><br><br><br>'.']]>'; ?></description>

Is that the proper use of the_excerpt? This doesn’t work for some reason.

Here is the complete file:

<?php
/*
Template Name: Custom Feed
*/

$numposts = 7;

function ffm_rss_date( $timestamp = null ) {
  $timestamp = ($timestamp==null) ? time() : $timestamp;
  echo date(DATE_RSS, $timestamp);
}
$posts = query_posts('showposts='.$numposts);

$lastpost = $numposts - 1;

header("Content-Type: application/rss+xml; charset=UTF-8");
echo '<?xml version="1.0"?>';
?><rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
<channel>
  <title>Website</title>
  <link>https://website.org/</link>
  <atom:link href="https://website.org/feed/ac" rel="self" type="application/rss+xml" />
  <description>The latest stories from Website</description>
  <language>en-us</language>
  <lastBuildDate><?php ffm_rss_date( strtotime($ps[$lastpost]->post_date_gmt) ); ?></lastBuildDate>
  <managingEditor>my@email.org (My name)</managingEditor>

  <item>
    <title><?php echo get_the_title($post->ID); ?></title>
    <link><?php echo get_permalink($post->ID); ?></link>
    <pubDate><?php ffm_rss_date( strtotime($ps[$lastpost]->post_date_gmt) ); ?></pubDate>
<description><?php echo '<![CDATA['.'<p><span>by </span><strong>'.get_post_meta($post->ID,'author',true).'</strong></p><br>'.the_excerpt_rss($post->ID).'<a href="'.get_permalink($post->ID).'">Read the rest of this story.</a><br><br><br>'.']]>';  ?></description>
    <guid><?php echo get_permalink($post->ID); ?></guid>
  </item>
<?php } ?>
</channel>
</rss>