Index: src/includes/users/functions.php
===================================================================
--- src/includes/users/functions.php	(revision 5424)
+++ src/includes/users/functions.php	(working copy)
@@ -512,7 +512,7 @@
 		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 );
 }
 
 /**