The value 'stable' is invalid for APT::Default-Release as such a release is not available in the sources

When apt-get upgrade-ing and you get the below error message,

# apt-get upgrade
Reading package lists... Done
E: The value 'stable' is invalid for APT::Default-Release as such a
release is not available in the sources

Please try commenting the line below in /etc/apt/apt.conf.d/70debconf .

APT::Default-Release "stable";