when i try to see the preview of the course units while

when i try to see the preview of the course units while usign the plugin coursepress, this message pops up at the top of the screen:

Warning: array_key_exists(): The first argument should be either a string or an integer in /home/wilmadc/public_html/courses/wp-content/plugins/coursepress/includes/classes/class.course.php on line 374