upgrade - Upgrading to Alpha fails due extras repositories

I try to upgrade to the latest release of Ubuntu (14.04 - Alpha).

I typed this in the Terminal:

$ sudo update-manager -d

After some checks, a popup appeared:

I clicked the Upgrade... button:

Preparing to upgrade was successful, but Setting new sofware channels fails:

How can this be fixed?

Just disable the "extra" repositories. These packages still not have trusty counter part, since most of them are for Stable versions:

To disable the extras repositories, just remove all the extras entries from /etc/apt/sources.list file or use the software sources and untick the extras entries there.

sudo sed -i '/extras\.ubuntu\.com/d' /etc/apt/sources.list