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 ('a81396a0c6db3e29a5d4c5f7a3b3819b', -1, 1455330205, 1455330205, '36c4d623', -1, 0, 0)

Line : 187
File : sessions.php