If you code in a check for time being greater than 6pm you can add the nt_, but you're right, its not in there to begin with. New versions of the API will have the correct nt_ in front of them. Sorry for the trouble!
I note that this issue is over two years ago, but I still see hourly icon not having 'nt_' in front of it. The icon_url does, but 'icon' does not. Is this really still an outstanding issue?!!?!? Since it is two years ago, is it ever going to get a fix?