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, is_robot, session_admin) VALUES ('df70f091664e7e591f4b4a5f9aaa0b87', -1, 1455406570, 1455406570, '36c4d92b', 7, 0, '0', 0)

Line : 174
File : sessions.php