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 ('6ff50907ed88d1d26c45a75d19f6f8fc', -1, 1438725252, 1438725252, '36904d1a', -1, 0, 0)

Line : 187
File : sessions.php