I have debug mode turned on in my Moodle 2.1,

In one of four courses, I go to the gradebook, then click on "User report". I get an error message about this "Undefined index." (see attached picture).

I just want to let you know about this. It doesn't seem to cause any error, it just shows itself in debug mode.

I tried this in all four of my courses, and this problem only presents itself in one. It is my smallest course, with only three students. So I am puzzled about this because it seems to have something to do with only one course. The problem may not be able to be repeated in your testing mode.