missing string while editing grade item category

Go to the gradebook, categories and items screen and click the edit icon next to a category. The following is displayed.

Invalid get_string() identifier: 'scaleid' or component 'grades'

• line 6139 of /lib/moodlelib.php: call to debugging()
• line 6691 of /lib/moodlelib.php: call to core_string_manager->get_string()
• line 145 of /grade/edit/tree/category_form.php: call to get_string()
• line 149 of /lib/formslib.php: call to edit_category_form->definition()
• line 93 of /grade/edit/tree/category.php: call to moodleform->moodleform()