WP_DEBUG warning

Testing this on a dev server with WP_DEBUG enabled, the following warning appears in Settings > WP Maintenance Mode > Modules >> Subcribe > Stats:

Notice: wpdb::prepare was called incorrectly. The query argument of wpdb::prepare() must have a placeholder. Please see Debugging in WordPress for more information. (This message was added in version 3.9.) in /var/www/myserver.com/html/wp-includes/functions.php on line 3560

It doesn’t seem to break any functionality at first glance, at least.

https://www.ads-software.com/plugins/wp-maintenance-mode/