zendframework/zf2 — pull request #3011 by pkarc (2012-11-19 06:52:11+00:00)
Title: Response Json Client

As http://www.json-rpc.org/wiki/specification

1.2 Response
error - An Error object if there was an error invoking the method. It must be null if there was no error.

Not always is an Error this parameter it could be null.
To be more explicit, when using json-client the response come with error as null if there is no error