The middleware didn't return a response or call next.

MiddlewareNoDataOrNextCalled: The middleware needs to either return a Response object or call the next function.

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;
});