I haven't installed Stretch quite yet, but I noticed that on https://www.debian.org/releases/stretch/errata : deb http://security.debian.org/ stretch main contrib non-free should be : deb http://security.debian.org/ stretch/updates main contrib non-free
The webpage http://www.debian.org/security/ has the correct line. I also checked https://www.debian.org/releases/jessie/errata and it's correct for that release. --Stosh
