diff --git a/docs_src/path_operation_advanced_configuration/tutorial004.py b/docs_src/path_operation_advanced_configuration/tutorial004.py
index fa867e794..da678aed3 100644
--- a/docs_src/path_operation_advanced_configuration/tutorial004.py
+++ b/docs_src/path_operation_advanced_configuration/tutorial004.py
@@ -11,7 +11,7 @@ class Item(BaseModel):
     description: Optional[str] = None
     price: float
     tax: Optional[float] = None
-    tags: Set[str] = []
+    tags: Set[str] = set()
 
 
 @app.post("/items/", response_model=Item, summary="Create an item")