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 ('c464dfd3015440d306b25c1b7145b899', -1, 1413886990, 1413886990, '36a1bffe', 2, 0, 0)

Line : 187
File : sessions.php