undefined constant WPS_PLUGIN

Hi,

I noticed the following message in my error log.

PHP Notice: Use of undefined constant WPS_PLUGIN - assumed 'WPS_PLUGIN' in /home/mywebsite/wp-content/plugins/wordpress-sentinel/wordpress_sentinel.php on line 322

Is this a sentinel issue? If so, how to fix?

Thanks,
Michiel

http://ww.wp.xz.cn/extend/plugins/wordpress-sentinel/