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 ('6f50b12e41a547026cd8fa44cd11756b', -1, 1455248003, 1455248003, '36a76054', 2, 0, '0', 0)

Line : 174
File : sessions.php