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 ('87bb4d7eb8122c6d6d98a86c6989f040', -1, 1414028125, 1414028125, '3210a53e', -1, 0, 0)

Line : 187
File : sessions.php