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 ('67f833afe97f7ab2e5b7b9d70d2d2f4d', -1, 1397876873, 1397876873, '36c4c6d5', 19, 0)

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