The following exception is most likely caused by not having the @Persistent annotation along with the @PrimaryKey annotation. Just a heads up.
The primary key for is an unencoded string but the key of the corresponding entity in the datastore does not have a name. You may want to either change the primary key to be an encoded string (add the "gae.encoded-pk" extension), change the primary key to be of type com.google.appengine.api.datastore.Key, or, if you're certain that this class will never have a parent, change the primary key to be of type Long.