When breaking down a character, what do you do for parts where no radical exists?
Actually if you can look at the traditional form of this character. The upper part is "丣".“丣” work as a phonetic radical here because it is pronounced you3. 

A lot of characters have been simplified in Mandarin in main land China. This make it harder for us to break down a character.

You can find more information here: 汉典解释 

So the first thing to do when breaking down a character is to find its traditional version.