Ticket #2651: 2651.01.patch

src/includes/users/functions.php

@@ -512 +512 @@
                 wp_cache_set( 'bbp_get_topic_subscribers_' . $topic_id, $users, 'bbpress_users' );
         }
 
-        return apply_filters( 'bbp_get_topic_subscribers', $users );
+        return apply_filters( 'bbp_get_topic_subscribers', $users, $topic_id );
 }
 
 /**