W: There is no public key available for the following key IDs error - Debian 7

When using apt-get update, you get this error:

# apt-get update
Ign cdrom://[Debian GNU/Linux 7.6.0 _Wheezy_ - Official i386 DVD Binary-1 20140712-13:02] wheezy Release.gpg
Ign cdrom://[Debian GNU/Linux 7.6.0 _Wheezy_ - Official i386 DVD Binary-1 20140712-13:02] wheezy Release
Ign cdrom://[Debian GNU/Linux 7.6.0 _Wheezy_ - Official i386 DVD Binary-1 20140712-13:02] wheezy/contrib i386 Packages/DiffIndex
Ign cdrom://[Debian GNU/Linux 7.6.0 _Wheezy_ - Official i386 DVD Binary-1 20140712-13:02] wheezy/main i386 Packages/DiffIndex
Ign cdrom://[Debian GNU/Linux 7.6.0 _Wheezy_ - Official i386 DVD Binary-1 20140712-13:02] wheezy/contrib Translation-en_GB
Ign cdrom://[Debian GNU/Linux 7.6.0 _Wheezy_ - Official i386 DVD Binary-1 20140712-13:02] wheezy/main Translation-en_GB
Hit http://security.debian.org wheezy/updates Release.gpg
Hit http://security.debian.org wheezy/updates Release
Hit http://security.debian.org wheezy/updates/main Sources
Hit http://security.debian.org wheezy/updates/contrib Sources
Hit http://security.debian.org wheezy/updates/main i386 Packages
Hit http://security.debian.org wheezy/updates/contrib i386 Packages
Hit http://security.debian.org wheezy/updates/contrib Translation-en
Hit http://security.debian.org wheezy/updates/main Translation-en
Reading package lists... Done 
W: There is no public key available for the following key IDs:
9D6D8F6BC857C906

Pull down the missing key, using:

# apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 9D6D8F6BC857C906
Executing: gpg --ignore-time-conflict --no-options --no-default-keyring --secret-keyring /tmp/tmp.bzDMKZFH54 --trustdb-name /etc/apt//trustdb.gpg --keyring /etc/apt/trusted.gpg --primary-keyring /etc/apt/trusted.gpg --keyring /etc/apt/trusted.gpg.d//debian-archive-squeeze-automatic.gpg --keyring /etc/apt/trusted.gpg.d//debian-archive-squeeze-stable.gpg --keyring /etc/apt/trusted.gpg.d//debian-archive-wheezy-automatic.gpg --keyring /etc/apt/trusted.gpg.d//debian-archive-wheezy-stable.gpg --keyserver keyserver.ubuntu.com --recv-keys 9D6D8F6BC857C906
gpg: requesting key C857C906 from hkp server keyserver.ubuntu.com
gpg: key C857C906: public key "Debian Security Archive Automatic Signing Key (8/jessie) " imported
gpg: no ultimately trusted keys found
gpg: Total number processed: 1
gpg:               imported: 1  (RSA: 1)
root@sonts-Mac-mini:~# apt-get update
Ign cdrom://[Debian GNU/Linux 7.6.0 _Wheezy_ - Official i386 DVD Binary-1 20140712-13:02] wheezy Release.gpg
Ign cdrom://[Debian GNU/Linux 7.6.0 _Wheezy_ - Official i386 DVD Binary-1 20140712-13:02] wheezy Release
Ign cdrom://[Debian GNU/Linux 7.6.0 _Wheezy_ - Official i386 DVD Binary-1 20140712-13:02] wheezy/contrib i386 Packages/DiffIndex
Ign cdrom://[Debian GNU/Linux 7.6.0 _Wheezy_ - Official i386 DVD Binary-1 20140712-13:02] wheezy/main i386 Packages/DiffIndex
Ign cdrom://[Debian GNU/Linux 7.6.0 _Wheezy_ - Official i386 DVD Binary-1 20140712-13:02] wheezy/contrib Translation-en_GB
Ign cdrom://[Debian GNU/Linux 7.6.0 _Wheezy_ - Official i386 DVD Binary-1 20140712-13:02] wheezy/main Translation-en_GB
Hit http://security.debian.org wheezy/updates Release.gpg
Hit http://security.debian.org wheezy/updates Release   
Hit http://security.debian.org wheezy/updates/main Sources
Hit http://security.debian.org wheezy/updates/contrib Sources
Hit http://security.debian.org wheezy/updates/main i386 Packages
Hit http://security.debian.org wheezy/updates/contrib i386 Packages
Hit http://security.debian.org wheezy/updates/contrib Translation-en
Hit http://security.debian.org wheezy/updates/main Translation-en
Reading package lists... Done