Install Plasma 5 KDE ubuntu - Cannot add PPA

I am trying to install Plasma 5 KDE on my ubuntu 14.04 with the following commands:

sudo apt-add-repository ppa:kubuntu-ppa/next-backports
sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get install kubuntu-plasma5-desktop plasma-workspace-wallpapers

It fails after the first command with the message:

 Cannot add PPA: 'ppa:kubuntu-ppa/next-backports'.
Please check that the PPA name or format is correct.

I got all from the following link

http://sourcedigit.com/14871-install-kde-plasma-5-2-ubuntu-14-04-ubuntu-14-10/

What is this error-message and how do I solve it?

ppa:kubuntu-ppa/next-backports is a valid name for a PPA, but there is no PPA with this name. Here is a list of valid PPAs of the “Kubuntu Package Archives” team.

• backports – Kubuntu Backports
• beta – Kubuntu Beta Backports
• experimental – Kubuntu Experimental
• ppa – Kubuntu Updates
• staging-frameworks – Kubuntu Staging Frameworks
• staging-kdeapplications – Kubuntu Staging KDE Applications
• staging-misc – Kubuntu Staging Misc
• staging-plasma – Kubuntu Staging Plasma

Therefore install an existing PPA, e.g.

sudo apt-add-repository ppa:kubuntu-ppa/backports