Custom Post Types not queried in "Link existing content"

In a post, linking to existing content displays custom post type posts. However, when using the search box, the results displayed only include Posts and Pages – and excludes my custom post types. Any ideas?

This is how I’m registering my post type:

register_post_type( 'journal',
		  array( 'label' => 'Journals',
		    'description' => 'Articles, videos, links, photo albums.',
		    'public' => true,
		    'show_ui' => true,
		    'publicly_queryable' => true,
		    'show_in_menu' => true,
		    'capability_type' => 'post',
		    'hierarchical' => false,
		    'rewrite' => array('slug' => 'journals', 'with_front'=>false, 'feeds' => true),
		    'query_var' => true,
		    'has_archive' => true,
		    'exclude_from_search' => false,
		    'menu_position' => 7,
		    'supports' => array(
		      'title',
		      'editor',
		      'excerpt',
		      'custom-fields',
		      'thumbnail',
		      'author',
		      ),
		    'taxonomies' => array(
		      // Custom Taxonomies are assigned with register_taxonomy()
		      ),
		    'labels' => array (
		      'name' => 'Journals',
		      'singular_name' => 'Journal',
		      'menu_name' => 'Journals',
		      'add_new' => 'Add Journal',
		      'add_new_item' => 'Add New Journal',
		      'edit' => 'Edit',
		      'edit_item' => 'Edit Journal',
		      'new_item' => 'New Journal',
		      'view' => 'View Journal',
		      'view_item' => 'View Journal',
		      'search_items' => 'Search Journals',
		      'not_found' => 'No Journals Found',
		      'not_found_in_trash' => 'No Journals Found in Trash',
		    ),
		  )
		);

It may be worth noting that similar functionality in the light-weight related-links plugin does find custom-post-types using the identical looking search.

Thanks!