Uncaught exception thrown in shutdown function.

PDOException: SQLSTATE[HY000]: General error: 2006 MySQL server has gone away: DELETE FROM {semaphore} WHERE (value = :db_condition_placeholder_0) ; Array ( [:db_condition_placeholder_0] => 558513570685f8804b7fcd1.29668746 ) in lock_release_all() (line 269 of /var/www/html/rtdrc/includes/lock.inc).