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) VALUES ('1cb246b8e70c6a5e604b0ddab006ecdd', -1, 1398405158, 1398405158, '36cdec2e', -1, 0)

Line : 158
File : /forum/includes/sessions.php