isinstance mocked pythonnet class

Question

I am making some python unit tests over classes which import C# with pythonnet, but mocks of those imported elements do not pass isinstance() tests.

Code under test:

import clr
clr.AddReference('MyDll')
from MyDll import ClassNet

def check(object_net):
    assert isinstance(object_net, ClassNet) # <-- fails
    ...

Test code:

import unittest
from unittest.mock import MagicMock
from MyCode import check

class Test(unittest.TestCase):
    def test(self):
        mock_object = MagicMock(spec=ClassNet)
        check(mock_object)

Test fails because isinstance return false although spec argument has been used. Based on Mock Doc:

If spec is an object (rather than a list of strings) then __class__ returns the class of the spec object. This allows mocks to pass isinstance() tests.

Any idea whether I am missing something?

Debugging, ClassNet is a CLR Metatype, but I don't know if this has something to do with it.