[Pit]

Разобрался: https://github.com/Microsoft/TypeScr...ess-properties

A property P is said to be expected in a type T if one of the following is true:

T is not an object, union, or intersection type.
T is an object type and
T has a property with the same name as P,
T has a string or numeric index signature,
T has no properties, or
T is the global type 'Object'.
T is a union or intersection type and P is expected in at least one of the constituent types of T.

Тип Object является исключением выделенным.