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 ('c8ea1f593f7e2a7e53f641f019f3946a', -1, 1411359980, 1411359980, '36a1c890', -1, 0, 0)

Line : 187
File : sessions.php