3.2 Upgrade

Just upgraded my site and received the following error:

Warning: require_once(/home/content/02/7969102/html/blog/wp-includes/class-json.php) [function.require-once]: failed to open stream: No such file or directory in /home/content/02/7969102/html/blog/wp-content/plugins/tweet-this/tweet-this.php on line 145

Fatal error: require_once() [function.require]: Failed opening required ‘/home/content/02/7969102/html/blog/wp-includes/class-json.php’ (include_path=’.:/usr/local/php5/lib/php’) in /home/content/02/7969102/html/blog/wp-content/plugins/tweet-this/tweet-this.php on line 145