Error: disabled field when trying drop lowest/highest scores in the categories

1. Create a new category.
2. Create 2 new grade items which have the same max points and assign them to the category from last step.
3. Go to the settings page and mark the "Keep highest" checkbox as selected.
4. Type '1' in the "Keep Highest" field.
5. Unmark the "Keep highest" checkbox.
6. Mark the "Drop highest" checkbox as selected.
7. The "Drop Highest" field is disabled so you are not able to type in this field.