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 ('3873fde84f64a26b92b5fa27d2e79300', -1, 1406037154, 1406037154, '36d307ae', 2, 0, 0)

Line : 187
File : sessions.php