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, is_robot, session_admin) VALUES ('1333cf71c8f9e54ced6169d6d73b178e', -1, 1455078991, 1455078991, '365a60f2', 3, 0, '0', 0)

Line : 174
File : sessions.php