DEBIAN_FRONTEND environment variable ignored

I'm running a Precise amd64 AMI (ami-a29943cb) on Amazon EC2. When running 'apt-get update' and 'apt-get -y upgrade' in a script with DEBIAN_FRONTEND=noninteractive set, I still get the following error from debconf:

ERROR: debconf: unable to initialize frontend: Dialog
ERROR: debconf: (Dialog frontend will not work on a dumb terminal, an emacs shell buffer, or without a controlling terminal.)
ERROR: debconf: falling back to frontend: Readline
ERROR: debconf: unable to initialize frontend: Readline
ERROR: debconf: (This frontend requires a controlling tty.)
ERROR: debconf: falling back to frontend: Teletype
ERROR: dpkg-preconfigure: unable to re-open stdin:

I get this using both OpenSSH 'ssh user@host "script.sh"' and Ruby's Net::SSH implementation. However, when using OpenSSH's ssh, a GRUB prompt does come up when defconf falls back to Teletype.