diff --git a/mypy/stubtest.py b/mypy/stubtest.py
index 9dd630d61d4a..5973d82f2383 100644
--- a/mypy/stubtest.py
+++ b/mypy/stubtest.py
@@ -2049,6 +2049,10 @@ def _named_type(name: str) -> mypy.types.Instance:
         return mypy.types.TupleType(items, fallback)
 
     fallback = mypy.types.Instance(type_info, [anytype() for _ in type_info.type_vars])
+    if type(runtime) != runtime.__class__:
+        # Since `__class__` is redefined for an instance, we can't trust
+        # its `isinstance` checks, it can be dynamic. See #20919
+        return fallback
 
     value: bool | int | str
     if isinstance(runtime, enum.Enum) and isinstance(runtime.name, str):
diff --git a/mypy/test/teststubtest.py b/mypy/test/teststubtest.py
index 4023a191b548..d373b1560271 100644
--- a/mypy/test/teststubtest.py
+++ b/mypy/test/teststubtest.py
@@ -1564,6 +1564,49 @@ def f(): return 3
             error=None,
         )
 
+    @collect_cases
+    def test_proxy_object(self) -> Iterator[Case]:
+        yield Case(
+            stub="""
+            class LazyObject:
+                def __init__(self, func: object) -> None: ...
+                def __bool__(self) -> bool: ...
+            """,
+            runtime="""
+            class LazyObject:
+                def __init__(self, func):
+                    self.__dict__["_wrapped"] = None
+                    self.__dict__["_setupfunc"] = func
+                def _setup(self):
+                    self.__dict__["_wrapped"] = self._setupfunc()
+                @property
+                def __class__(self):
+                    if self._wrapped is None:
+                        self._setup()
+                    return type(self._wrapped)
+                def __bool__(self):
+                    if self._wrapped is None:
+                        self._setup()
+                    return bool(self._wrapped)
+            """,
+            error="test_module.LazyObject.__class__",
+        )
+        yield Case(
+            stub="""
+            def default_value() -> bool: ...
+
+            DEFAULT_VALUE: bool
+            """,
+            runtime="""
+            def default_value():
+                return True
+
+            DEFAULT_VALUE = LazyObject(default_value)
+            bool(DEFAULT_VALUE)  # evaluate the lazy object
+            """,
+            error="test_module.DEFAULT_VALUE",
+        )
+
     @collect_cases
     def test_all_at_runtime_not_stub(self) -> Iterator[Case]:
         yield Case(