Problems with Get-Compliance

I can't seem to figure out why this is failing:

> $ESXHost = Get-VMHost esx006

> get-compliance -entity $ESXHost -detailed

get-compliance : 5/31/2013 3:01:45 PM    Get-Compliance        API version '5.1' for vCenter Server

'<vCenter>' is not supported.

At line:1 char:1

+ get-compliance -entity $esxhost -detailed

+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    + CategoryInfo          : NotSpecified: (:) [Get-Compliance], VimException

    + FullyQualifiedErrorId : Core_BaseCmdlet_UnknownError,VMware.VumAutomation.Commands.GetCompliance

Searching is not bringing up anything. Any ideas? Update Manager is installed on the vCenter. I am connected to the vCenter when running the script.