Why is Astro site not building with Contentful fields?

Question

I have an Astro site that uses Contentful as a CMS.

I want to fetch data from Contentful to display on the website. I've been following the guidance on the Astro site.

lib/contentful.ts

import contentful, { type EntryFieldTypes, type Asset } from "contentful";

export interface homepage {
  contentTypeId: "page",
  fields: {
    introductionTitle: EntryFieldTypes.Text
    introductionSubtitle: EntryFieldTypes.Text
    heroImage: Asset
  }
}

Then display the results on index.astro

import { contentfulClient } from "../lib/contentful";
import type { homepage } from "../lib/contentful";

const entries = await contentfulClient.getEntries<homepage>({
  content_type: "page",
});

const pages = entries.items.map((item) => {
  const { introductionTitle, introductionSubtitle, heroImage } = item.fields;
  return {
    introductionTitle,
    introductionSubtitle,
    heroImage
  };
});

This works great on the development server, however when I go to build the site locally or deploy the site to Vercel I get the following error:

▶ src/pages/index.astro └─ /index.htmlCannot read properties of undefined (reading 'fields')