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 ('c749fc80eaf5f7e3a764b0ec2c07302f', -1, 1406237975, 1406237975, '3650645a', -1, 0, 0)

Line : 187
File : sessions.php