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 ('60789b1427fdee0b6201bb5d22cea5e1', -1, 1480955943, 1480955943, '369e773c', 28, 0, 0)

Line : 172
File : sessions.php