class Item(BaseModel):
    name: str
    price: Optional[float] = None
    owner_ids: Optional[List[int]] = None

app = FastAPI()

@app.get("/items/invalidnone", response_model=Item)
def get_invalid_none():
    return None
```

...calling the path `/items/invalidnone` will raise an error, because `None` is not a valid type for the `response_model` declared with `Item`.