RSS.xml.js returns `<title>ReferenceError</title>` with content collection content | Astro Lounge | Page 1

I'm trying to make an RSS feed for my site, loosly following the Add an RSS Feed guide. It says that include the post content, you need a third-party Markdown parser like Markdown-IT, but earlier today I used post.render() in content collections, which seemed to return plain HTML, so I tried to use that instead. Here is the file:

// rss.xml.js
import rss from "@astrojs/rss";
import { getCollection } from "astro:content";
renderedPost = await post.render()

export async function get() {
  const posts = await getCollection('blog');
  return rss({
    title: 'Southpaw1496',
    description: 'My Blog',
    site: 'https://southpaw1496.me/',
    items: posts.map((post) => ({
      title: post.data.title,
      pubDate: post.data.pubDate,
      description: post.data.description,
      link: `/blog/${post.slug}/`,
      content: renderedPost,
    })),
    customData: `<language>en-gb</language>`,
  });
};

When I try to use this file, running curl localhost:4321/rss.xml, it returns

<title>ReferenceError</title><script type="module" src="/@vite/client"></script>%

Why might this be happening?

Astro Documentation

Add an RSS feed

Add an RSS feed to your Astro site to let users subscribe to your content.

#RSS.xml.js returns `<title>ReferenceError</title>` with content collection content