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?

从链接的 NewType 文档部分开始，并使用 ObjectId 重现该诊断。阐明“可正确进行子类化的类”是什么意思，并解释为什么 ObjectId 被报告为 Any，然后验证修订后的措辞是否处理了该示例并保留现有的 NewType 指导。