Gradebook

My teachers are getting an unusual eror. This error ONLY ever occurs, if they first click on the gradebook, then click on view grads. If they then go to one of the categories, and then click on one of the students that is NOT in their group, they get this error. This seems to be the only time it occurs. The error reads:

Warning: ksort() expects parameter 1 to be array, null give in /home/webadmin/childbirthinternational.com/html/moodle/grade/lib.php on line 1872.

I checked the grade/lib.php file, and line 1872 reads:

// displays all grades for the course

Any ideas wqhy this odd error would be occurring only in these very specific circumstances?

Nikki