David M. on "Unregister a widget area without id"

I want to unregister a widget area provided by a parent theme. This is what the Codex advises:

<?php unregister_sidebar( $id ); ?>

And this is how the parent theme registers the sidebar:

register_sidebar(array(
	'name' => 'Pages Sidebar',
	'description' => esc_html__('A widget area', 'parent-theme'),
));

unregister_sidebar works with the id, but the theme registers the sidebar without an id. What can I do to unregister that? Is there a default id, and how could I find it out?

Thanks!