juju deploys wrong machine when no machine matching constraints is available

This should have failed

lucky(~/src/launchpad.net/juju-core/worker/uniter/jujuc) % juju deploy --repository=$HOME/charms --constraints='cpu-cores=9000' local:raring/gccgo gccgo2

lucky(~/src/launchpad.net/juju-core/worker/uniter/jujuc) % juju status gccgo2
environment: ap-southeast-2
machines:
  "2":
    agent-state: pending
    instance-id: i-680ccb54
    instance-state: pending
    series: raring
    hardware: arch=amd64 cpu-cores=1 cpu-power=100 mem=1740M root-disk=8192M
services:
  gccgo2:
    charm: local:raring/gccgo-12
    exposed: false
    units:
      gccgo2/0:
        agent-state: pending
        machine: "2"

But it didn't, it deployed a smaller machine.