Recent custom post widget issues.

I've installed the plugin, and it works fine without Thumbnails , but once I put the images option on, I get this error:

Any ideas?

Fatal error: Call to undefined function get_post_thumbnail_id() in /nfs/c07/h02/mnt/102542/domains/diktacratie.com/html/wp-content/plugins/recent-custom-post-type/recent-custom-post-type.php on line 188