diff --git a/fastapi/routing.py b/fastapi/routing.py
index fa1351859..2375b389d 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -502,7 +502,11 @@ class APIRoute(routing.Route):
                     additional_status_code
                 ), f"Status code {additional_status_code} must not have a response body"
                 response_name = f"Response_{additional_status_code}_{self.unique_id}"
-                response_field = create_response_field(name=response_name, type_=model)
+                response_field = create_response_field(
+                    name=response_name,
+                    type_=model,
+                    mode="serialization",
+                )
                 response_fields[additional_status_code] = response_field
         if response_fields:
             self.response_fields: Dict[Union[int, str], ModelField] = response_fields