Index: src/includes/common/functions.php
===================================================================
--- src/includes/common/functions.php	(revision 5404)
+++ src/includes/common/functions.php	(working copy)
@@ -1222,13 +1222,13 @@
 		$topic_url
 	);
 
-	$message = apply_filters( 'bbp_forum_subscription_mail_message', $message, $topic_id, $forum_id, $user_id );
+	$message = apply_filters( 'bbp_forum_subscription_mail_message', $message, $topic_id, $forum_id, null );
 	if ( empty( $message ) ) {
 		return;
 	}
 
 	// For plugins to filter titles per reply/topic/user
-	$subject = apply_filters( 'bbp_forum_subscription_mail_title', '[' . $blog_name . '] ' . $topic_title, $topic_id, $forum_id, $user_id );
+	$subject = apply_filters( 'bbp_forum_subscription_mail_title', '[' . $blog_name . '] ' . $topic_title, $topic_id, $forum_id, null );
 	if ( empty( $subject ) ) {
 		return;
 	}