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 ('b6a3ffd0c5d308aa06735c6c4397b2cf', -1, 1398361922, 1398361922, '6b1543bf', 16, 0)

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