How to store a nodes Fivestar average rating inside another field with computed fields
Eventualy I used a different methode to get the average value. Here's how

 //Get average rating and place in index review score
    $average_field = field_view_field('node', $entity, 'field_review_votes_average');
    $rating = $average_field['#items'][0]['average'];
$entity-&gt;field_index_review_score['und'][0]['value'] = $rating;