Change BIOS boot order

Hi

I want to change the boot order for my virtual machine. This script I found doesn't work. Any idea why?

$spec=New-Object Vmware.Vim.VirtualMachineConfigSpec
$spec.extraConfig += new-object Vmware.Vim.OptionValue
$spec.extraConfig[0].key="bios.bootDeviceClasses"
$spec.extraConfig[0].value="allow:cd,hd"
get-vm|%{(get-view $_.ID).ReconfigVM_Task($spec)}