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 ('df2b0f7ed18a6ed94ec004bd265bb14b', -1, 1398242366, 1398242366, '36c4c448', 6, 0)

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