kit: hooks.js example causes a build error

Describe the bug

When the following hooks.js file exists, npm run build throws an error. (However npm run dev works without error and the site runs perfectly.)

To Reproduce

1. Add src/hooks.js

export async function handle(request, render) {
  const response = await render(request);

  return {
    ...response,
    headers: {
      ...response.headers,
      'x-custom-header': 'potato'
    }
  };
}

1. Run npm run build.

The command will throw this error:

✓ 38 modules transformed.
.svelte/output/server/app.js      193.07kb
.svelte/output/server/style.css   7.76kb

Run npm start to try your app locally.

> Using @sveltejs/adapter-node
TypeError: Cannot read property 'headers' of undefined
    at Object.handle (file:///Users/me/sveltekit-demo/.svelte/output/server/app.js:1890:19)
    at async ssr (file:///Users/me/sveltekit-demo/.svelte/output/server/app.js:1677:12)
    at async visit (file:///Users/me/sveltekit-demo/node_modules/@sveltejs/kit/dist/chunks/index5.js:531:20)
    at async prerender (file:///Users/me/sveltekit-demo/node_modules/@sveltejs/kit/dist/chunks/index5.js:682:5)
    at async Builder.prerender (file:///Users/me/sveltekit-demo/node_modules/@sveltejs/kit/dist/chunks/index5.js:743:4)
    at async adapt (/Users/me/sveltekit-demo/node_modules/@sveltejs/adapter-node/index.js:26:4)
    at async adapt (file:///Users/me/sveltekit-demo/node_modules/@sveltejs/kit/dist/chunks/index5.js:765:2)
    at async file:///Users/me/sveltekit-demo/node_modules/@sveltejs/kit/dist/cli.js:598:5
> 500 /about
Error: 500 /about
    at error (file:///Users/me/sveltekit-demo/node_modules/@sveltejs/kit/dist/chunks/index5.js:523:11)
    at visit (file:///Users/me/sveltekit-demo/node_modules/@sveltejs/kit/dist/chunks/index5.js:574:5)
    at async prerender (file:///Users/me/sveltekit-demo/node_modules/@sveltejs/kit/dist/chunks/index5.js:682:5)
    at async Builder.prerender (file:///Users/me/sveltekit-demo/node_modules/@sveltejs/kit/dist/chunks/index5.js:743:4)
    at async adapt (/Users/me/sveltekit-demo/node_modules/@sveltejs/adapter-node/index.js:26:4)
    at async adapt (file:///Users/me/sveltekit-demo/node_modules/@sveltejs/kit/dist/chunks/index5.js:765:2)
    at async file:///Users/me/sveltekit-demo/node_modules/@sveltejs/kit/dist/cli.js:598:5
npm ERR! code 1
npm ERR! path /Users/me/sveltekit-demo
npm ERR! command failed
npm ERR! command sh -c svelte-kit build

about.svelte is nothing special:

<h1>About this site</h1>
<p>TODO...</p>

svelte.config.cjs contains adapter as: adapter: node(),.

Information about your SvelteKit Installation:

  System:
    OS: macOS 11.2.2
    CPU: (8) arm64 Apple M1
    Shell: 5.8 - /bin/zsh
 npmPackages:
    @sveltejs/kit: next => 1.0.0-next.64 
    svelte: ^3.35.0 => 3.35.0 

Severity

Minor