Generating localized sitemap fails with TypeError: Cannot read properties of null (reading 'childNodes')

Link to the code that reproduces this issue

https://codesandbox.io/p/devbox/mystifying-newton-42cjqp?file=%2Fapp%2Fsitemap.ts

To Reproduce

Use the example from documentation

import type { MetadataRoute } from 'next'

export default function sitemap(): MetadataRoute.Sitemap {
  return [
    {
      url: 'https://acme.com',
      lastModified: new Date(),
      alternates: {
        languages: {
          es: 'https://acme.com/es',
          de: 'https://acme.com/de',
        },
      },
    },
    {
      url: 'https://acme.com/about',
      lastModified: new Date(),
      alternates: {
        languages: {
          es: 'https://acme.com/es/about',
          de: 'https://acme.com/de/about',
        },
      },
    },
    {
      url: 'https://acme.com/blog',
      lastModified: new Date(),
      alternates: {
        languages: {
          es: 'https://acme.com/es/blog',
          de: 'https://acme.com/de/blog',
        },
      },
    },
  ]
}

yarn next dev
Browser console throws Uncaught TypeError: Cannot read properties of null (reading 'childNodes')

Current vs. Expected behavior

The sitemap.xml should normally render correctly, but it renders plain text: https://acme.com 2024-05-15T20:48:04.146Z https://acme.com/about 2024-05-15T20:48:04.146Z https://acme.com/blog 2024-05-15T20:48:04.146Z. The error happens only if I set the alternates attribute! Without alternates it work's like a charm.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.2.0
  Available memory (MB): 16384
  Available CPU cores: 10
Binaries:
  Node: 18.19.0
  npm: 10.2.3
  Yarn: 1.22.19
  pnpm: N/A
Relevant Packages:
  next: 14.2.3
  eslint-config-next: 14.2.3
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.4.5
Next.js Config:
  output: standalone

Which area(s) are affected? (Select all that apply)

Metadata

Which stage(s) are affected? (Select all that apply)

next dev (local)

Additional context

No response

We could not detect a valid reproduction link. Make sure to follow the bug report template carefully.

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vercel / next.js