Gradebook

I think the error is caused by a legacy, no grade assignment from the original course which was created in 1.9.3. What happened is that when we upgraded, non graded assignments stopped displaying in the gradebook (a bug). But they did display in 1.9.3 and still do in that restored course which causes the error?

I can't find anything else different but the course was created on a previous version and remained a course when we updated.

I wonder if upgrading to another version fixes the issue...

UPDATE: restoring to 1.9.5 took care of the issue. So it's a Moodle version error caused by the above (thankfully rare) phenomena.