Displaying taxonomy as category page?

I’d like to display every post tagged with a certain taxonomy on a page. Here’s what I’ve got so far…

In functions.php, I’ve created a custom post type and custom taxonomy:

// IMAGE GALLERY
	register_post_type(
		  'gallery', array(
			  'labels' => array(
				  'name' => 'Image Gallery',
				  'singular_name' => 'Gallery Image',
				  'add_new' => 'Add new gallery image',
				  'add_new_item' => 'Add gallery image',
				  'new_item' => 'New gallery image',
				  'view_item' => 'View gallery images',
				  'edit_item' => 'Edit gallery image',
				  'not_found' =>  __('No gallery images found'),
				  'not_found_in_trash' => __('No gallery images found in Trash')
			  ),
			  'public' => true,
			  'publicly_queryable' => true,
			  'show_ui' => true,
			  'query_var' => true,
			  'rewrite' => true,
			  'capability_type' => 'post',
			  'exclude_from_search' => true,
			  'hierarchical' => false,
			  'menu_position' => null,
			  'supports' => array('title', 'thumbnail'),
			  'taxonomies' => array('image_category')
		 )
	  );

// Custom Taxonomies
function asw_register_taxonomies() {
	register_taxonomy("image_category", array("gallery"),
	array(
		"hierarchical" => true,
		"label" => __('Image Categories', 'gallery'),
		"singular_label" => "Image Category",
		"query_var" => true,
		"rewrite" => true));
}

The taxonomies show up perfectly in the post type and are successfully saved with the post.

Then I have a template for the gallery which contains this code for the taxonomy list:

<?php // LIST IMAGE CATEGORIES
$categories = array(
	'show_option_all'    => '',
	'orderby'            => 'name',
	'order'              => 'ASC',
	'style'              => 'list',
	'show_count'         => 0,
	'hide_empty'         => 1,
	'use_desc_for_title' => 1,
	'child_of'           => 0,
	'feed'               => '',
	'feed_type'          => '',
	'feed_image'         => '',
	'exclude'            => '',
	'exclude_tree'       => '',
	'include'            => '',
	'hierarchical'       => true,
	'title_li'           => __( '' ),
	'show_option_none'   => __('No categories'),
	'number'             => null,
	'echo'               => 1,
	'depth'              => 0,
	'current_category'   => 0,
	'pad_counts'         => 0,
	'taxonomy'           => 'image_category',
	'walker'             => null
); ?>

<?php wp_list_categories( $categories ); ?>

…which works like a charm, displaying all the categories of the taxonomy. But when I click on one of the links in the list, it sends me to [url]/image_category/sample-images/ — which uses index.php and contains a “page not found” error.

I have a template named taxonomy-image_category.php which contains a standard loop, but WP isn’t using it; it loads the category from index.php instead. I even tried creating a generic taxonomy.php page, but no cigar. Does anyone know what I’m missing?