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 ('5f77b88e729742acad8d337042e7821f', -1, 1475347957, 1475347957, '365b0b0b', -1, 0, 0)

Line : 187
File : sessions.php