Remove Memory Limits On Guest VM's?

Hi All,

I'm having a bit of trouble running this script, I'm using the latest version of vSphere 5.1 and PowerCLI

Get-Cluster "My Cluster" | Get-VM | Get-VMResourceConfiguration | where {$_.MemLimitMB -ne "-1"} | Set-VMResourceConfiguration -MemLimitMB $null

I get the following error:

Set-VMResourceConfiguration : 7/05/2013 5:44:08 PM    Set-VMResourceConfigurati

on        Permission to perform this operation was denied.

At line:1 char:139

+ Get-Cluster "My Cluster" | Get-VM | Get-VMResourceConfiguration |

where {$_.MemLimitMB -ne "-1"} | Set-VMResourceConfiguration <<<<  -MemLimitMB

$null

    + CategoryInfo          : NotSpecified: (:) [Set-VMResourceConfiguration],

    NoPermission

    + FullyQualifiedErrorId : Client20_VmServiceImpl_SetVMResourceConfiguratio

   n_ViError,VMware.VimAutomation.ViCore.Cmdlets.Commands.SetVMResourceConfig

  uration