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 ('3b3b0f4ea9c68f4cb22fddcad5a95102', -1, 1398301575, 1398301575, '43ca3870', 19, 0)

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