phpBB : Critical Error

Error creating new session

DEBUG MODE

SQL Error : 1062 Duplicate entry '-1' for key 'session_user_id'

INSERT INTO sessions (session_id, session_user_id, session_start, session_time, session_ip, session_page, session_logged_in, session_admin) VALUES ('26d26d383c050f0f1e8e1c505afc9a7b', -1, 1425571091, 1425571091, '3691b5e6', 17, 0, 0)

Line : 187
File : sessions.php