Error when attempting to preview units in CoursePress

I receive the following error when attempting to preview courses or units. All plugins and theme are up to date.

Warning: array_key_exists(): The first argument should be either a string or an integer in /home4/divasrul/public_html/cordsandchords.com/wp-content/plugins/coursepress/includes/classes/class.course.php on line 374

Thoughts?