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 ('cc0c78392129da74e508eae3d7409e87', -1, 1417013409, 1417013409, '36a1c082', 0, 0, 0)

Line : 187
File : sessions.php