Are you sure the private key is valid? It should be long and start with a 5. For example:
5JRp9CTGhGcih9VUMV1yvACjS5UxJgEaXhU3MD51Fyc3BeNr2Sw
Absolutely it's valid. I did it on two different computers same result. Don't get me wrong it works fine. I get the balance on the key and I can send and receive no problem.
The issue is that:
1.) THe response in the console that I assume should probably say "Success" or something is blank.
2.) The public address appears in the address book instead of my receiving addresses until a restart of bitcoin. Then it moves over where it belongs and everything works as normal.