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 ('7e0c8476dccbb558e78a1786599a69b5', -1, 1406070815, 1406070815, '36e343af', -1, 0, 0)

Line : 187
File : sessions.php