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 ('38dffa46a7cfad1e4899e5fbcff588f9', -1, 1411025939, 1411025939, '36a6088a', -1, 0, 0)

Line : 187
File : sessions.php