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 ('d7bd5dc6c61fd006c15a6a3013502f0f', -1, 1438725349, 1438725349, '3211577e', 3, 0, 0)

Line : 187
File : sessions.php