Trying to use blog excerpt as meta description for blog

I'm not sure what I'm missing but I'm getting no output in my generated html markup for meta description.

blog/post.php has this

<title><?php perch_blog_post_field(perch_get('s'), 'postTitle'); ?></title>
    <meta name="description" content="<?php 
    perch_blog_custom(array(
        'filter'=>'postSlug',
        'match'=>'eq',
        'value'=>perch_get('s'),
        'template'=>'excerpt.html',
    ));
    ?>">

In post.html the excerpt field looks like this:

<perch:blog id="excerpt" type="textarea" label="Excerpt" html="true" order="3" suppress="true" size="s" />

And I have a blog template excerpt.html that looks like this:

<perch:blog id="excerpt" type="textarea" label="Excerpt"  html="true" striptags="true" />

After I save my blog post, the html then looks like this:

<title>Welcome to my new website</title>
    <meta name="description" content="">

I'd welcome any ideas!

Summary information

    Perch: 2.8.15, PHP: 5.4.33, MySQL: mysqlnd 5.0.10 - 20111026 - $Id: c85105d7c6f7d70d609bb4c000257868a40840ab $, with PDO
    Server OS: Linux, cgi-fcgi
    Installed apps: content (2.8.15), assets (2.8.15), categories (2.8.15), perch_blog (5.0), perch_gallery (2.8.6)
    App runtimes: <?php $apps_list = array( 'content', 'categories', 'perch_gallery', 'perch_blog', );
    PERCH_LOGINPATH: /admin
    PERCH_PATH: /home/amodeowe/public_html/tw/admin
    PERCH_CORE: /home/amodeowe/public_html/tw/admin/core
    PERCH_RESFILEPATH: /home/amodeowe/public_html/tw/admin/resources
    Image manipulation: GD
    PHP limits: Max upload 40M, Max POST 20M, Memory: 128M, Total max file upload: 20M
    Resource folder writeable: Yes
    HTTP_HOST: www.tw.hardingweb.net
    DOCUMENT_ROOT: /home/amodeowe/public_html/tw
    REQUEST_URI: /admin/core/settings/diagnostics/
    SCRIPT_NAME: /admin/core/settings/diagnostics/index.php