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 ('68b7301618f5b5d034eee573c5c16f7a', -1, 1469928740, 1469928740, '365cb8de', -1, 0, 0)

Line : 187
File : sessions.php