Enom domain purchases fail with error message in Api log

I'm using pro-sites with enom domain mapping. I've jumped through all the normal Enom hoops and got everything setup, managed to successfully map a domain I already own to a multisite install using the plugin.
When I'm testing purchasing a new domain, although payment is taken (via PayPal) the purchase fails with this error message visible in the Api log:
"User Defined Extended attribute: registered_for is required for registered_for"
Another thread recommended checking that Enom default period length had been set, and indeed it has been set for a year by default on my Enom account.
Any other suggestions?