diff --git a/sanic_openapi/doc.py b/sanic_openapi/doc.py
index cf255d94..5f12aebe 100644
--- a/sanic_openapi/doc.py
+++ b/sanic_openapi/doc.py
@@ -140,7 +140,9 @@ def definition(self):
             "properties": {
                 key: serialize_schema(schema)
                 for key, schema in chain(
-                    {key: getattr(self.cls, key) for key in dir(self.cls)}.items(),
+                    {
+                        key: getattr(self.cls, key, None) for key in dir(self.cls)
+                    }.items(),
                     typing.get_type_hints(self.cls).items(),
                 )
                 if not key.startswith("_")