Ticket #23668: 23668.2.patch

src/wp-includes/taxonomy.php

@@ -352 +352 @@
         }
 
         if ( false !== $args['rewrite'] && ( is_admin() || '' != get_option( 'permalink_structure' ) ) ) {
+                if ( empty( $args['rewrite']['slug'] ) )
+                        $args['rewrite']['slug'] = sanitize_title_with_dashes( $taxonomy );
                 $args['rewrite'] = wp_parse_args( $args['rewrite'], array(
-                        'slug' => sanitize_title_with_dashes( $taxonomy ),
                         'with_front' => true,
                         'hierarchical' => false,
                         'ep_mask' => EP_NONE,