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 ('9203ec3c2066c6a7b7c327db6d82078c', -1, 1475235852, 1475235852, '36c491ad', -1, 0, 0)

Line : 187
File : sessions.php