Preview custom post type posts

I’m having difficulty previewing draft posts from within custom post types. If I click the ‘preview’ button it just sends me to the front page of the site.

If I create a custom post type with the custom post type ui plugin (instead of defining it manually in the functions.php file as I’m doing now) then I can preview them so I’m obviously missing something in the register_post_type declaration – i just can’t figure out what.

My code looks like this:

register_post_type( 'press_releases',
		array(
			'labels' => array(
				'name' => __( 'Press Releases' ),
				'singular_name' => __( 'Press Release' ),
				'add_new_item' => __( 'Add New Press Release' )
			),
			'menu_position' => 5,
			'public' => true,
			'rewrite' => array('slug' => 'press-releases', 'with_front' => true),
			'supports' => array('title', 'editor', 'excerpt', 'thumbnail' ),
			'capabilities' => array(
				'edit_post' => 'edit_press_release',
				'edit_posts' => 'edit_press_releases',
				'edit_others_posts' => 'edit_others_press_releases',
				'publish_posts' => 'publish_press_releases',
				'read_post' => 'read_press_release',
				'read_private_posts' => 'read_private_press_releases',
				'delete_post' => 'delete_press_release',
			)
		)
	);

The obvious get-out clause for this would be to define the the post types through the plugin but:

1. I have around 10 post types already defined and it would be a total pain to re-create them.
2. I need to assign separate user capabilities to each post type, something which the plugin doesn’t do.
3. I’d really like to know for future reference.

Thanks for your help