I have registered settings and need it to be showed in REST API.

$args = [
    'show_in_rest' => true
];
register_setting('default_sidebars', 'default_sidebars', $args);

When I save a single value and then request data on endpoint /wp-json/wp/v2/settings, everything works perfectly.

But the problem is that I save serialized data like this.

a:2:{s:4:"post";s:7:"general";s:4:"blog";s:9:"sidebar_1";}

Now I would expect in response something like this:

default_sidebars: {
  post: "general",
  blog: "sidebar_1"
}

But instead I got default_sidebars: null.

What should I do to get my data in REST API?