FastAPI - Error Handling
Find the bug in this FastAPI code that tries to return a custom error response:
```python
from fastapi import FastAPI, HTTPException
from pydantic import BaseModel
class ErrorResponse(BaseModel):
message: str
app = FastAPI()
@app.get("/users/{user_id}", responses={404: {"model": ErrorResponse}})
async def get_user(user_id: int):
if user_id == 0:
return ErrorResponse(message="User not found")
return {"user_id": user_id}
```