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 ('55e1c470207b4ee5ad2077e2c50b6d7b', -1, 1406879241, 1406879241, '36598aee', -1, 0, 0)

Line : 187
File : sessions.php