Warning: A non-numeric value encountered

I'm getting this warning in my Watchdog log when viewing my list of referers:

Warning: A non-numeric value encountered in Drupal\visitors\Controller\Report\Referers->_getData() (line 144 of /var/www/mysiteweb/modules/contrib/visitors/src/Controller/Report/Referers.php).

Here's the code in question:

143     $page = isset($_GET['page']) ? (int) $_GET['page'] : '';
144     $i = 0 + $page * $items_per_page;

The issue here is that $page can get set to a string value if the expression evaluates to FALSE. Here's the fix:

143     $page = isset($_GET['page']) ? (int) $_GET['page'] : 0;