Can't upgrade enchantments with /enchant

Steps to reproduce:
1. Give yourself a diamond sword, for example.
2. Use "/enchant @p unbreaking" to enchant it with Unbreaking I.
3. Now use "/enchant @p unbreaking 3" to enchant it with Unbreaking III.

Expected result:
It should just upgrade Unbreaking I to Unbreaking III. You can combine Unbreaking I with Unbreaking III on anvils, /enchant should be able to do it too.

Actual result:
"Unbreaking III can't be combined with Unbreaking I"