How to make matlab response with "not an object"

I've created a CNN for flower recognition with live testing, but even if there is nothing in front of webcam it will still response with something like a rose when there is only an empty wall. I would like to response with "not a flower". How can i do this? I tried adding a class named "not a flower" with objects, faces,cars etc but the results are not so great. Is there something i can do?