Build fails with "Error: You must supply options.input to rollup"

[Malte Jürgens]

I just discovered microsite and wanted to try it out, but I get this error:

> yarn create microsite microsite-test
> cd microsite-test
> yarn
> yarn build
yarn run v1.22.10
$ microsite build
[17:03:44] [snowpack] Hint: run "snowpack init" to create a project config file. Using defaults...
[17:03:44] [snowpack] ! building files...
[17:03:44] [snowpack] ✔ files built. [0.06s]
[17:03:44] [snowpack] ! building dependencies...
[17:03:45] [snowpack] ✔ dependencies built. [0.29s]
[17:03:45] [snowpack] ! writing to disk...
[17:03:45] [snowpack] ✔ write complete. [0.01s]
[17:03:45] [snowpack] ▶ Build Complete!
(node:9756) UnhandledPromiseRejectionWarning: Error: You must supply options.input to rollup
    at Graph.generateModuleGraph (file:///C:/Users/malte/Programme/microsite-test/node_modules/rollup/dist/es/shared/rollup.js:19253:19)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
    at async Graph.build (file:///.../microsite-test/node_modules/rollup/dist/es/shared/rollup.js:19194:9)
    at async rollupInternal (file:///.../microsite-test/node_modules/rollup/dist/es/shared/rollup.js:20165:9)
    at async bundlePagesForSSR (file:///.../microsite-test/node_modules/microsite/dist/cli/microsite-build.js:144:20)
    at async Promise.all (index 0)
    at async build (file:///.../microsite-test/node_modules/microsite/dist/cli/microsite-build.js:51:33)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:9756) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 2)
(node:9756) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
Done in 1.70s.

Am I doing something wrong here or is this a bug?

OS: Windows 10
Node version: v14.16.0
I get the same error when using npm instead of yarn