SetItem does not respect null about svelte-persistent-store HOT 1 OPEN

<script>
  import { writable } from 'svelte/store';
  import * as local from 'svelte-persistent-store/dist/local';

  const test =
    process.browser
      ? local.writable('test', null)
      : writable(null);
</script>

<style>
</style>

<p>
  {JSON.stringify($test)}
  {$test === null}
  <!-- {$test === undefined} -->
  <button on:click={() => test.set("foo")}>foo</button>
  <button on:click={() => test.set(null)}>null</button>
  <!-- <button on:click={() => test.set(undefined)}>undefined</button> -->
</p>