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 ('d2e6ea6a1a06aa68370947c7544dc5ec', -1, 1406893825, 1406893825, '17141e3f', 42, 0, 0)

Line : 180
File : sessions.php