Smarter navigation with thumbnails / get_adjacent_id_smart not working

I’m trying to do something similar to this thread but I’m not having any luck.

I tried stripping this down to the essentials, and I discovered that when I echo get_adjacent_id_smart(true) or get_adjacent_id_smart(false), I always get -1. See example:

<p><?php previous_post_smart();?> | <?php next_post_smart();?></p>
<?php $prev_post_id = get_adjacent_id_smart( true ); echo $prev_post_id; ?>

Using the default previous/next_post_smart links works just fine, but the get_adjacent_id_smart always just returns -1 .

I’m using this on a custom post type.

Any help is greatly appreciated!

http://wordpress.org/plugins/smarter-navigation/