Really large economies don’t go bust. The highest ranked economies to enter default, according to a Bank of Canada database, stood at around 10th: Brazil and Mexico in the 1980s, Russia in 1991. China, in contrast, has the biggest economy (on some measures) ever seen. Yet Moody’s is fretting and has marked down its outlook to “negative” from “stable”.

It is easy to scoff. China’s debts may be high but are denominated in renminbi, and its reserves are the largest anywhere. But everything about China is unprecedented: its size and recent growth, and the scale of its challenges. Moody’s cites large contingent liabilities to banks and local government, and the difficulty Beijing faces managing currency convertibility.

The odds against default are overwhelming. Still, landslides start with a single pebble. China’s Aa3 rating is associated with a 1 in 1,200 chance of default. In light of its challenges, this is no overestimate.