In one of the page's getStaticPaths the return value had the incorrect shape.

Make sure to return the following shape from getStaticPaths:

export async function getStaticPaths() {
  return {
    paths: Array<string | { params: { [key: string]: string } }>,
    fallback: boolean
  }
}

There are two required properties:

This property is an Array of URLs ("paths") that should be statically generated at build-time. The returned paths must match the dynamic route shape.

• You may return a String or an Object that explicitly defines all URL params.

  export async function getStaticPaths() {
    return {
      paths: [
        // String variant:
        '/blog/first-post',
        // Object variant:
        { params: { slug: 'second-post' } },
      ],
      fallback: true,
    }
  }

This property can be a Boolean, specifying whether or not a fallback version of this page should be generated, or a string 'blocking' to wait for the generation:

• Enabling fallback (via true) allows you to return a subset of all the possible paths that should be statically generated. At runtime, Next.js will statically generate the remaining paths the first time they are requested. Consecutive calls to the path will be served as-if it was statically generated at build-time. This reduces build times when dealing with thousands or millions of pages.
• Disabling fallback (via false) requires you return the full collection of paths you would like to statically generate at build-time. At runtime, any path that was not generated at build-time will 404.
• If fallback is 'blocking', new paths not returned by getStaticPaths will wait for the HTML to be generated, identical to SSR (hence why blocking), and then be cached for future requests so it only happens once per path.