How to properly alter response from Solr Search Module
The problem was this one:

function zen_search_api_solr_search_results_alter(array &amp;$results, SearchApiQueryInterface $query, Apache_Solr_Response $response){
  ...
}


It should be this:

function zen_search_api_solr_search_results_alter(&amp;$results, $query, $response){
  ...
}