Custom taxonomy “hierarchical’ => true” not working in WordPress 2.8

Another WordPress issue from me!

I’ve been trying to set up two custom taxonomies in WordPress 2.8 for “Course subject” and “Type of opportunity”.

I used this code in functions.php:

function create_pc_db_taxonomies() {
    register_taxonomy( 'course', 'post', array(
        'hierarchical' => false,
        'label' => __('Course subject', 'series'),
        'query_var' => 'course',
        'rewrite' => array( 'slug' => 'courses' )
    ) );

    register_taxonomy( 'type', 'post', array(
        'hierarchical' => false,
        'label' => __('Type of opportunity', 'series'),
        'query_var' => 'type',
        'rewrite' => array( 'slug' => 'types' )
    ) );
}

which works absolutely fine, but I want hierarchical (category-style) admin boxes rather than tag-style admin boxes.

However, when I set 'hierarchical' => true so that the above code becomes:

function create_pc_db_taxonomies() {
    register_taxonomy( 'course', 'post', array(
        'hierarchical' => true,
        'label' => __('Course subject', 'series'),
        'query_var' => 'course',
        'rewrite' => array( 'slug' => 'courses' )
    ) );

    register_taxonomy( 'type', 'post', array(
        'hierarchical' => true,
        'label' => __('Type of opportunity', 'series'),
        'query_var' => 'type',
        'rewrite' => array( 'slug' => 'types' )
    ) );
}

I get no boxes at all.

Am I doing something wrong?