How to get a custom user profile property in people manager rest api?

In Sharepoint 2013, I have a custom user profile property, and I can't seem to get it with

    http://mysite/_api/SP.UserProfiles.PeopleManager/GetMyProperties?$select=CustomNumber

But if I try this, it gets all the properties including the one I want.

    http://mysite/_api/SP.UserProfiles.PeopleManager/GetMyProperties?$select=UserProfileProperties

Somehow, UserProfileProperties evaluated to all the properties...

Is there a way I can do a further filter and select the one I want?

Thanks