I can't preview a Unit page in CoursePress Pro. When

I can't preview a Unit page in CoursePress Pro. When clicking on Preview I get the error message below:

Warning: array_key_exists(): The first argument should be either a string or an integer in /Applications/MAMP/htdocs/guitar-studio/wp-content/plugins/coursepress/includes/classes/class.course.php on line 374
30 NOV -0001

I have included a screen shot of my editor showing the code from the class.course .php file.