Use author_can() on custom post types?

I’m trying to conditionally display some content in a theme using custom roles and capabilities. First, I define a custom role:

add_role('free_vendor', 'Free Vendor Listing', array('read', 'edit_posts', 'delete_posts', 'display_map'));

This is run directly from functions.php (do I need to add it to an action somewhere?)

Then I use the display_map capability I defined to conditionally display the map:

if (author_can($post, 'display_map')) echo '<li><a href="#map">Map</a></li>';

The only problem is, it doesn’t work! Is there some issue in using author_can() with custom post types? Am I not calling add_role() properly?