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 ('dfd751e4f6525a4f443d94e50e3e2c80', -1, 1472350277, 1472350277, '36e0a02a', -1, 0, 0)

Line : 187
File : sessions.php