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?

Commencez par la section liée de la documentation de NewType et reproduisez le diagnostic avec ObjectId. Clarifiez ce que signifie « classe correctement dérivable » et expliquez pourquoi ObjectId est signalé comme Any, puis vérifiez que la formulation révisée traite l’exemple et préserve les indications existantes sur NewType.