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 ('5e4f52d3283c4a884a04ffe05ab3b19f', -1, 1419229024, 1419229024, '36ed97bc', 8, 0, 0)

Line : 187
File : sessions.php