Uncaught exception thrown in session handler.

PDOException: SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '0' for key 'uid': INSERT INTO {sessions} (sid, ssid, uid, cache, hostname, session, timestamp) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6); Array ( [:db_insert_placeholder_0] => o8j1YaoUtfD2UNgr58GmOpTneArbbPlpFybA9hjmb5Q [:db_insert_placeholder_1] => [:db_insert_placeholder_2] => 0 [:db_insert_placeholder_3] => 0 [:db_insert_placeholder_4] => 216.73.216.26 [:db_insert_placeholder_5] => themekey_theme|s:2:"mm"; [:db_insert_placeholder_6] => 1775163591 ) in _drupal_session_write() (line 209 of /var/www/gothic.hu/includes/session.inc).