When looking up the profile for the username "_" (a single underscore), the API returns an invalid UUID. The provided UUID is 3126ba4c6uud424c877d1347fa974d23. However, UUIDs are only supposed to contain digits and the letters A-F; this ID contains the letter U. This causes code using the API, including the official authentication library, to crash.
This can be reproduced by going to the following link: https://api.mojang.com/users/profiles/minecraft/_