Can't change my name when it's not used yet

I want to change my MC username so i checked namemc.com to see if it was used and it wasn't, so when i tried to change it it didn't let me. Actually, they don't seem to accept any name with "asuka" in it, because i tried gibberish with asuka in the middle of it (within the 16 carachter limit) and nothing worked. It's a really weird thing, im not even sure if it's a bug or not.