upgrade to 2.0

I did all like in upgrade instruction. When I press button Upgrade on site/install page come blank page and no any actions

in install error log
[20-Oct-2014 07:44:50 America/New_York] PHP Fatal error:  Class 'DB\stdClass' not found in /home/donbassb/public_html/system/library/db/mysql.php on line 39