Domain Mapping plugin is giving "Domain name is invalid" error

We have a new domain, threehouse.org, that we recently pointed to our multisite install's IP address, 72.14.178.34. The domain is resolving to the IP appropriately as the browser redirects to the multisite's main site, aboundant.com. In the Domain Mapping plugin, when we try to input the domain to be mapped we are getting a "Domain name is invalid" error.

Also, WP Engine is our host and we have added the domain in their list of domains attached to this WordPress install.

Any ideas?