phpBB : Critical Error

Error creating new session

DEBUG MODE

SQL Error : 1062 Duplicate entry '-1' for key 2

INSERT INTO phpbb_sessions (session_id, session_user_id, session_start, session_time, session_ip, session_page, session_logged_in, session_admin) VALUES ('f6ce974941e7a6da39d1fd0d15ee57c9', -1, 1397664756, 1397664756, '6ca2d4f8', -4, 0, 0)

Line : 187
File : sessions.php