I can't add domain

[hostingstark]

Hello.

Any can help me?

I am trying add a domain. The domain is "domain.com" and this:

"Your Domain name is not valid. Please enter a valid Domain Name: i.e. 'domain.com"

The domain is valid but i can't insert. Later if i insert others domain with other name i can insert, for example "otherdomain.com".

Thanks.