diff --git a/fastapi/routing.py b/fastapi/routing.py
index c97668a75c..d043d809d6 100644
--- a/fastapi/routing.py
+++ b/fastapi/routing.py
@@ -979,9 +979,7 @@ class APIRoute(routing.Route):
 
     @property
     def is_json_stream(self) -> bool:
-        return self.is_generator and isinstance(
-            self.response_class, DefaultPlaceholder
-        )
+        return self.is_generator and isinstance(self.response_class, DefaultPlaceholder)
 
     @cached_property
     def _flat_dependant(self) -> Dependant:
@@ -1013,7 +1011,6 @@ class APIRoute(routing.Route):
         _ = self.response_fields
         _ = self.stream_item_field
 
-
     def get_route_handler(self) -> Callable[[Request], Coroutine[Any, Any, Response]]:
         return get_request_handler(
             dependant=self.dependant,
diff --git a/tests/test_route_deferred_init.py b/tests/test_route_deferred_init.py
index 17e9274259..0514eee8fe 100644
--- a/tests/test_route_deferred_init.py
+++ b/tests/test_route_deferred_init.py
@@ -11,6 +11,7 @@ deferred_keys = [
     "response_fields",
     "body_field",
     "response_field",
+    "stream_item_field",
     "dependant",
     "_flat_dependant",
     "_embed_body_fields",