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, session_admin) VALUES ('846b95e083853b0e845921be4d6eb563', -1, 1409648765, 1409648765, 'b8489a7c', -1, 0, 0)

Line : 187
File : sessions.php