Why does jsonb_set() remove non-mentioned keys?

Why does 

   select jsonb_set('{"foo": 1}'::jsonb, '{bar}', to_jsonb(null::int), true)

return NULL when all it should do is to add a second key?

I would expect {"foo": 1, "bar": null} or no change at all to the original JSON value, but not that the whole JSON is
setto null. 
 

In the original case the new value to be set was the result of an expression, not a "hardcoded" null value.
 
Thomas