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 ('5d94981c9c1beddd45c30008f7a169d6', -1, 1417147286, 1417147286, '36ea07a1', 6, 0, 0)

Line : 191
File : sessions.php