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 ('2a2c1979ebbc87d23a39b434685e8788', -1, 1472005853, 1472005853, '36914cba', 26, 0, 0)

Line : 187
File : sessions.php