orderby error

I’ve been debugging my theme and keep getting this error for my results:

Notice: Undefined index: orderby in /mysite/www/wp-content/plugins/yet-another-related-posts-plugin/class-cache.php on line 122

Here’s what I’m using. Order is the default score but I’ve tried other parameters getting the same error. I don’t use the default category or post_tag, so I’ve had to explicitly state my new taxonomies in the arguments below.

yarpp_related(array(
				            'post_type' => array('column','video','cartoon'),
				            'template' => 'yarpp-template-riotwire-vertical-thumbs.php', 

				             'require_tax' => array(
                                  'topic' => 0 //is this correct?
                              ),
				              'weight' => array(
                                     'body' => 1,
                                     'title' => 2, // larger weights mean this criteria will be weighted more heavily
                                     'tax' => array(
                                        'location' => 1,
                                        'topic' => 4,
                                        'issue' => 3,
                                      )
                              ),
                              'threshold' => 1,   // The threshold which must be met by the "match score"
				             'limit' => 3,
				             'order' => 'score DESC'

				            )
				      );
           }

http://wordpress.org/extend/plugins/yet-another-related-posts-plugin/