It's quite controversial to list that as an exception as many consider it incorrect, same as its use in questions ('did he use to' vs. 'did he used to') since simply treating it as any other verb (to use) makes more sense than treating it as a non-changeable entity 'used to'. Some dictionaries will include an exception for it but even those will usually also mention that it's not advisable to use that form in formal writing or written exams.
It's quite controversial to list that as an exception as many consider it incorrect, same as its use in questions ('did he use to' vs. 'did he used to') since simply treating it as any other verb (to use) makes more sense than treating it as a non-changeable entity 'used to'. Some dictionaries will include an exception for it but even those will usually also mention that it's not advisable to use that form in formal writing or written exams.