Default Manager and Teacher role unable to manually mark course as completed

When enabling course completion, Teacher and Manager role are unable to manually mark course as completed.

To reproduce in Moodle 2.5+ (Build: 20130530):

• fresh Moodle install - all roles at their defaults
• create a course with completion enabled
• set course completion condition "Manual completion by others" checking manager or teacher role
• crate a user "MNG" and assign Manager or teacher role in the course
• create another user "STD" and assign him the student role
• login as the "MNG" user and access the course
• access the course completion report and try to mark as complete the course for user "STD"
• an error message appears, related to the user's completion not being tracked

The same error appears for the Teacher role, also in Moodle 2.4.

A possible workaround is altering the manager or teacher role setting the capability "isincompletionreport" to Allow. However that way Manager and Teacher will appear in the completion report, which may not make sense in many usage scenario.