Domain Mapping- Entering a domain

For the Domain Mapping plugin, I have "supporter" set to "no".

When a client browses to Tools>Domain Mapping, he can type in the domain name, but when he presses "Add" the domain does not save. There is no error message - the page reloads as if no text was entered.

If the Super Admin performs the same action, the domain successfully saves.

How can I allow clients to enter their own domain names?