What would you expect in this case? Objects/abstract.c:2429 is where the isinstance code. If only returning False would be enough, something like this (untested) would be enough

--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -2436,6 +2436,10 @@ object_isinstance(PyObject *inst, PyObject *cls)
         retval = PyObject_TypeCheck(inst, (PyTypeObject *)cls);
         if (retval == 0) {
             retval = _PyObject_LookupAttrId(inst, &PyId___class__, &icls);
+            if (retval == NULL && PyErr_Occurred()) {
+                PyErr_Clear();
+                retval = 0;
+            }