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 ('9d29637f3db6954edfcc0e52ee825a2a', -1, 1397780371, 1397780371, '36e38de6', -1, 0, 0)

Line : 187
File : sessions.php