installation failed (12.04) - ubiquity crash

when I tried to install ubuntu 12.04, I got this error.

"/usr/lib/ubiquity/plugins/ubi-timezone.py" follows the message "ubiquity crashed with ValueError in raw_decode(): No JSON object could be decoded"

this is the discussion of ubuntu forum : "http://ubuntuforums.org/archive/index.php/t-1965802.html"

and I fixed this that unplugged network cable.