To re-create:

• Create an assignment
• Create a rubric for the assignment
• Enter in criteria and items into the rubric, each with point value of 1
• Grade a student's submission, and the grade will not be updated (grade == -1)

Instead of just allowing the user to do this and not updating the grade, please add a validation error when saving the rubric to prevent this scenario. Also, give a warning/error when grading to let the user know that the rubric is invalid and why the grade isn't saving.