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app = FastAPI()

@app.get("/items/invalidnone", response_model=Item)
def get_invalid_none():
    if flag:
        return {"name": "foo"}
    # if flag is False, at this point the function will implicitly return None
```

If you have *path operations* using `response_model` that need to be allowed to return `None`, make it explicit in `response_model` using `Union[Something, None]`: