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) VALUES ('936226689d802c2b79d82ca1f112296d', -1, 1398045604, 1398045604, '36f31781', 11, 0)

Line : 158
File : /forum/includes/sessions.php