Error Building EC2onRails

[Eric]

Hi.
I'm trying to build a custom AMI using EC2onRails and following the
instructions in the README.
I start a developer AMI on EC2 and then copy over all my files and
start the build. It works fine until it finishing extracting the
packages and then crashes while running the "chroot /mnt/build/ubuntu
mount -t proc proc /proc" command.
Has anyone else run into this or know why it would be happening?
Here is an excerpt of the output:

I: Extracting ncurses-base...
I: Extracting ncurses-bin...
I: Extracting passwd...
I: Extracting perl-base...
I: Extracting procps...
I: Extracting python-minimal...
I: Extracting python2.6-minimal...
I: Extracting sed...
I: Extracting startup-tasks...
I: Extracting system-services...
I: Extracting sysv-rc...
I: Extracting sysvinit-utils...
I: Extracting tar...
I: Extracting tzdata...
I: Extracting upstart...
I: Extracting upstart-compat-sysv...
I: Extracting upstart-logd...
I: Extracting util-linux...
I: Extracting zlib1g...
W: Failure trying to run: chroot /mnt/build/ubuntu mount -t proc proc /
proc

Cheers,
Eric Goodwin

[Paul Dowman]

What are you building it on? I usually use one of Amazon's official AMI's: ami-f51aff9c.