Follow Post (User) Not Working

I am trying to subscribe the assigning editor of a post (as stored in an EditFlow custom metadata field), but can’t get the follow_post_user function to work properly. I haven’t had problems with the similar code for subscribing groups though.

Here is my code. Any suggestions would be appreciated!

function dfm_auto_subscribe_assigning_editor( $new_status, $old_status, $post ) {
	global $edit_flow;

	$dfm_upcoming_editor_id = get_post_meta($post->ID,'_ef_editorial_meta_user_editor', true);
	if ($dfm_upcoming_editor_id != null) {
		$edit_flow->notifications->follow_post_user( $post->ID, $dfm_upcoming_editor_id, true );
	}
	// Return true to send the email notification
	return $new_status;
}
add_filter( 'ef_notification_status_change', 'dfm_auto_subscribe_assigning_editor', 10, 3 );

http://wordpress.org/extend/plugins/edit-flow/