Out of interest, what is the domain name you are trying to add? .com/.net/.org/.biz domains can't be less than 6 characters (as they have a minimum of 2 letters before the extension) and I can only think of "bl.uk" which is a valid domain less than 6 characters (and that's the only domain name that I can think of which is 5 characters or less: and that's a "one-off" domain - most .uk domains are in the format .co.uk/.me.uk/.gov.uk etc - with nic.uk being another "special case", but that's 6 characters).