"unserialize(): Error at offset 0 of 166 " on my Wordpress Piwik widget

Hi everyones, I have a WordPress and I've installed Piwik on my server. Piwik works with "VisitGenerator" but not in my Wp-Piwik widget in Wordpress, I recieve this error :

Notice: unserialize(): Error at offset 0 of 166 bytes in /www/wordpress/wp-content/plugins/wp-piwik/wp-piwik.php on line 925

My Php-level is very close from Zero, could you help please ?

Thx (:P)