Fix fatal error.

main/inc/lib/zombie/zombie_manager.class.php

@@ -28,6 +28,9 @@ class ZombieManager
      */
     static function listZombies($ceiling, $active_only = true, $count = 0, $from = 10, $column = 'user.firstname', $direction = 'desc')
     {
+        if (empty($column)) {
+            $column = 'user.firstname';
+        }
         $ceiling = is_numeric($ceiling) ? (int) $ceiling : strtotime($ceiling);
         $ceiling = date('Y-m-d H:i:s', $ceiling);
 

main/inc/lib/zombie/zombie_report.class.php

@@ -209,6 +209,7 @@ class ZombieReport implements Countable
 
         $ceiling = $this->get_ceiling();
         $active_only = $this->get_active_only();
+
         $items = ZombieManager::listZombies($ceiling, $active_only, $count, $from, $column, $direction);
         $result = array();
         foreach ($items as $item) {