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 ('7e8e7773e1eb2c476a1c97bb225150c1', -1, 1455475966, 1455475966, '3690504b', 12, 0, '0', 0)

Line : 174
File : sessions.php