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 ('f9423796cf5aeb1a5a9f87eae748b2d7', -1, 1398416703, 1398416703, '36cc446d', 2, 0)

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