XML-RPC: trying to transition to the new wp.GetPosts

I currently pull the last few posts from my blog using this code:

$client = new IXR_Client($XMLRPC);

        if (!$client->query('metaWeblog.getRecentPosts',$BLOGID, $USERID,$PASSWORD,$NUMPOSTS)) {
            die('An error occurred - '.$client->getErrorCode().":".$client->getErrorMessage());
        }
        $response = $client->getResponse();

I recently noticed there are new wp methods introduced, so decided to transition from metaWeblog.getRecentPosts to wp.getPosts. From the Codex it appeared they take the same parameters so I tried simply replacing the call. Unfortunately that didn’t work.

An error occurred – 403:The post type specified is not valid

I’ve tried Googling for this, and for a working example of wp.getPosts being used, and have found nothing useful. Is this a simple case of not supplying the correct parameters? A simple example would be most useful.

Thanks in advance!