Downloading http://cdn-fastly.deb.debian.org/debian/dists/stretch-backports/InRelease...

gpgv:  RSA key ID B7D453EC

gpgv:  RSA key ID 22F3D138

Downloading http://cdn-fastly.deb.debian.org/debian/dists/stretch-backports/Release...
Downloading http://cdn-fastly.deb.debian.org/debian/dists/stretch-backports/Release.gpg...

gpgv:  RSA key ID B7D453EC

gpgv:  RSA key ID 22F3D138

ERROR: unable to fetch mirror: verification of detached signature failed: exit status 2