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 ('c7c18686f3cbe8c42874b30df94d9520', -1, 1440920846, 1440920846, '36e2db56', -1, 0, 0)

Line : 187
File : sessions.php