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 ('c038c4942c60e2567e0f42ff606e98aa', -1, 1485285719, 1485285719, '3693fd2d', -1, 0, 0)

Line : 187
File : sessions.php