phpBB : Critical Error

Error creating new session

DEBUG MODE

SQL Error : 1062 Duplicate entry '-1' for key 'session_user_id'

INSERT INTO phpbb_sessions (session_id, session_user_id, session_start, session_time, session_ip, session_page, session_logged_in, session_admin) VALUES ('efa6baa0257d83b3dfe8bd5b5f989d08', -1, 1406608548, 1406608548, '3653e0cb', 15, 0, 0)

Line : 180
File : sessions.php