A mysterious increase of radiocarbon 14C ca. 775 AD in the Earth’s atmosphere has been recently found by Miyake et al. (Miyake, F., Nagaya, K., Masuda, K., Nakamura, T. [2012]. Nature, 486, 240). A possible source of this event has been discussed widely, the most likely being an extreme solar energetic particle event. A new exotic hypothesis has been presented recently by Liu et al. (Liu, Y. [2014]. Sci. Rep., 4, 3728) who proposed that the event was caused by a cometary impact on Earth bringing additional 14C to the atmosphere. Here we calculated a realistic mass and size of such a comet to show that it would have been huge (≈100km across and 1017–1020g of mass) and would have produced a disastrous geological/biological impact on Earth. The absence of an evidence for such a dramatic event makes this hypothesis invalid.