@@ -272,7 +272,9 @@ mod _abc {
         let impl_data = get_impl(&cls, vm)?;
 
         // Get instance.__class__
-        let subclass = instance.get_attr("__class__", vm)?;
+        let subclass = vm
+            .get_attribute_opt(&instance, "__class__")?
+            .unwrap_or_else(|| instance.class().to_owned().into());
 
         // Check cache
         if in_weak_set(&impl_data.cache, &subclass, vm)? {
 
@@ -36,3 +36,28 @@ class SubConcrete(Concrete):
 assert isinstance(Concrete(), CustomInterface)
 assert isinstance(SubConcrete(), CustomInterface)
 assert not isinstance((), CustomInterface)
+
+
+class NoClassAttribute:
+    @property
+    def __class__(self):
+        raise AttributeError("no class attribute")
+
+
+instance = NoClassAttribute()
+assert not isinstance(instance, CustomInterface)
+CustomInterface.register(NoClassAttribute)
+assert isinstance(instance, CustomInterface)
+
+class_error = RuntimeError("class lookup failed")
+
+
+class BrokenClassAttribute:
+    @property
+    def __class__(self):
+        raise class_error
+
+
+with assert_raises(RuntimeError) as caught:
+    isinstance(BrokenClassAttribute(), CustomInterface)
+assert caught.exception is class_error