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 ('4fe9e4efb6789040a7bd0d3751e62f7e', -1, 1397654052, 1397654052, '36cc86b7', 0, 0, 0)

Line : 187
File : sessions.php