Notice: Undefined index: value in visitors_get_mysql_current_timezone() (line 242 sites/all/modules/visitors/visitors.module)

I don't know if that is really a bug or a problem of my host server (linux server, Apache version 2.0.64, PHP version 5.2.9)...

When i go to the visitors stats (eg visitors/monthly_history) the following notice appears without displaying the graph :

Notice: Undefined index: value in visitors_get_mysql_current_timezone() (line 242 sites/all/modules/visitors/visitors.module)

I changed the line 242 :
return $data['value'];

to :
return $data['Value'];

and the problem solved.