Wordpress Custom hook problem

custom_hook_client.php

class hook_client {

   static function start() {
      //logging here shows that this function is being called
      add_filter('custom_filter', array(__CLASS__, 'filter_handler'), 1, 2 );
      //logging here shows that the custom_filter hook is being hooked to
   }

   static function filter_handler($content, $postdata) {

      //logging here shows that this method is never entered
      //do stuff with $content

      return $content

   }

}

hook_client::start();

custom_hook_plugin.php

class plugin {

   function do_stuff() {
      
      // do some stuff with $content
      $postdata['content'] = apply_filters('custom_filter', $content, &$postdata);
      // logging $postdata['content'] returns null here
   }

}