Need help with Tumblr Settings?

I recently made a Tumblr, and I was messing around with the settings. I put a domain in the url section of the Settings, not realizing that you have to already own one, and now when I try to revert back to the regular "name.tumblr.com" it doesn't work. What do I do? Now whenever I try to see my page, it says it cant find the domain (I've train unchecking the domain setting, but when I save, it goes back).