The middleware returned something that is not a Response object.

MiddlewareNotAResponse: Any data returned from middleware must be a valid Response object.

What went wrong?

Section titled What went wrong?

Thrown in development mode when middleware returns something that is not a Response object.

For example:

import {defineMiddleware} from "astro:middleware";
export const onRequest = defineMiddleware(() => {
  return "string"
});