"Error while executing fastboot flash system" when installing Ubuntu on Nexus 4

When installing Ubuntu on my Nexus 4 device I got the following error:

Device detected as mako
Starting new HTTP connection (1): cdimage.ubuntu.com
Download uri set to http://cdimage.ubuntu.com/ubuntu-touch/daily-preinstalled/20130722
Download directory set to /home/waleed/Downloads/phablet-flash/ubuntu-touch/20130722
Retrieving files
Restarting device... wait
Restarting device... wait complete
The device needs to be unlocked for the following to work
Flashing system to /home/waleed/Downloads/phablet-flash/ubuntu-touch/20130722/saucy-preinstalled-system-armel+mako.img
< waiting for device >
erasing 'system'...
FAILED (remote: not supported in locked device)
finished. total time: 0.000s
Error while executing fastboot flash system /home/waleed/Downloads/phablet-flash/ubuntu-touch/20130722/saucy-preinstalled-system-armel+mako.img
Make sure the device is connected and viewable by running 'adb devices'
Ensure you have a root device, one which running 'adb root' does not return an error

The phone restarts and enters fast boot menu.

Any idea how to get past this problem?