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 ('b6d2cc768da150c8266ee6b5bb51d973', -1, 1414257975, 1414257975, '36a346f9', 0, 0, 0)

Line : 373
File : sessions.php