ExceptionFactory for 404

[zh355245849]

Hi, guys,
How can I use the ExceptionFactory to override 404 code with null result? I have tried below but none of these can work, it always throw exception when code is not 404:
ExceptionFactory = httpResponse => httpResponse.StatusCode != HttpStatusCode.NotFound ? Task.FromResult<Exception?>(null) : (Task<Exception?>)Task.CompletedTask,

ExceptionFactory = httpResponse => httpResponse.StatusCode != HttpStatusCode.NotFound ? Task.FromResult<Exception?>(null) : null

Can someone guide me to the correct path?
Thanks,
Han