Why do I get "Only absolute URLs are supported" when I just use API Routes on the same base url?

The Problem

I have a simple next.js application. On the index site I have following code:

export async function getServerSideProps(ctx: NextPageContext) {
  const json = await myGet("/api/people", ctx);
  return {
    props: {
      people: json,
    }, // will be passed to the page component as props
  };
}

myGet does just a simple fetch (with isomorphic fetch) to an api route with the url /api/people. This however fails with this error:

Server Error TypeError: Only absolute URLs are supported

My expectation would be that I can use a path just like "/api/people" since I serve the app from the same base url. In next.js do I have to explicitly provide the full URL? (e.g. https://my-app/api/people)

Thx guys