The middleware didn't return a Response.

MiddlewareNoDataOrNextCalled: Make sure your middleware returns a Response object, either directly or by returning the Response from calling the next function.

What went wrong?

Section titled What went wrong?

Thrown when the middleware does not return any data or call the next function.

For example:

import {defineMiddleware} from "astro:middleware";
export const onRequest = defineMiddleware((context, _) => {
  // doesn't return anything or call `next`
  context.locals.someData = false;
});