apt-get update: GPG error

# apt-get update

W: GPG error: http://ftp.de.debian.org stable/non-US Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F1D53D8C4F368D5D W: You may want to run apt-get update to correct these problems

KEY_TO_ADD="F1D53D8C4F368D5D"
echo $(gpg --keyserver subkeys.pgp.net --recv-keys $KEY_TO_ADD)
echo $(gpg --armor --export $KEY_TO_ADD | apt-key add -)
apt-get update