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 ('7a473e01da66aaf002fad3871a0e7d85', -1, 1398282024, 1398282024, '36ead9f7', 6, 0)

Line : 158
File : /home/gtclub/public_html/gtc/forum/includes/sessions.php