diff --git a/src/Solr/SolrHelper.php b/src/Solr/SolrHelper.php
index 1afbd22..ee38d62 100644
--- a/src/Solr/SolrHelper.php
+++ b/src/Solr/SolrHelper.php
@@ -748,7 +748,6 @@ class SolrHelper {
   public function setSorts(Query $solarium_query, QueryInterface $query, $field_names = array()) {
     $new_schema_version = version_compare($this->getSchemaVersion(), '4.4', '>=');
     foreach ($query->getSorts() as $field => $order) {
-      $f = '';
       // The default Solr schema provides a virtual field named "random_SEED"
       // that can be used to randomly sort the results; the field is available
       // only at query-time.
@@ -757,27 +756,27 @@ class SolrHelper {
         // Random seed: getting the value from parameters or computing a new
         // one.
         $seed = !empty($params['seed']) ? $params['seed'] : mt_rand();
-        $f = 'random_' . $seed;
+        $sort_field = 'random_' . $seed;
       }
       elseif ($field == 'search_api_relevance') {
-        $f = $field_names[$field];
+        $sort_field = $field_names[$field];
       }
       elseif ($new_schema_version && (strpos($field_names[$field], 't') === 0 || strpos($field_names[$field], 's') === 0)) {
         // For fulltext fields use the dedicated sort field for faster alpha
         // sorts. Use the same field for strings to sort on a normalized value.
-        $f = 'sort_' . $field;
+        $sort_field = 'sort_' . $field;
       }
       elseif ($new_schema_version && preg_match('/^([a-z]+)m(_.*)/', $field_names[$field], $matches)) {
         // For multi-valued fields (which aren't sortable by nature) we use
         // the same hackish workaround like the DB backend: just copy the
         // first value in a single value field for sorting.
-        $f = $matches[1] . 's' . $matches[2];
+        $sort_field = $matches[1] . 's' . $matches[2];
       }
       else {
-        $f = $field_names[$field];
+        $sort_field = $field_names[$field];
       }
 
-      $solarium_query->addSort($f, strtolower($order));
+      $solarium_query->addSort($sort_field, strtolower($order));
     }
   }