"John of Nikiû's Chronicle was originally written mostly in Greek except possibly some of the chapters concerning Egypt which may have been written in Coptic to judge from the forms of the names. The work only survives in an Ethiopian translation made in 1602 of an Arabic translation of the original. The text is in parts obviously corrupted with unfortunate accidental omissions. Most notably, a passage covering 30 years from 610 to 640 are missing"--Wikipedia.