Clarification of `properly subclassable class` in NewType

I want to create a new type derived from ObjectId in similar way as is described in documentation for new type UserId derived from int:

from bson import ObjectId
UserId = NewType('UserId', ObjectId)

But Mypy gives Argument 2 to NewType(...) must be subclassable (got "Any").

There is written in documentation:

NewType accepts exactly two arguments. The first argument must be a string literal containing the name of the new type and must equal the name of the variable to which the new type is assigned. The second argument must be a properly subclassable class, i.e., not a type construct like Union, etc.

It is not clear what properly subclassable class means and why ObjectId is not properly subclassable class. Could please someone clarify this to me and in documentation?

リンク先のNewTypeドキュメントのセクションから始め、ObjectIdを使って診断を再現してください。「適切にサブクラス化可能なクラス」が何を意味するのかを明確にし、ObjectIdがAnyとして報告される理由を説明してください。そのうえで、改訂された文言が例に対応し、既存のNewTypeに関するガイダンスを維持していることを確認してください。