Astro.locals is not serializable

LocalsNotSerializable: The information stored in Astro.locals for the path ”HREF” is not serializable. Make sure you store only serializable data. (E03034)

What went wrong?

Section titled What went wrong?

Thrown in development mode when a user attempts to store something that is not serializable in locals.

For example:

import {defineMiddleware} from "astro/middleware";
export const onRequest = defineMiddleware((context, next) => {
  context.locals = {
    foo() {
      alert("Hello world!")
    }
  };
  return next();
});