Get-WmiObject -Namespace root\wmi -Class wmiMonitorID : Incompatible ?????

Hi,

I'm working with a powershell script and this instruction usually works well:

Get-WmiObject -Namespace root\wmi -Class wmiMonitorID 

But in one pc it doesn't... the message is:

Get-WmiObject : Incompatible 

+     $ActiveMonitors = Get-WmiObject <<<<  -Namespace root\wmi -Class wmiMonitorID
+ CategoryInfo          : InvalidOperation: (:) [Get-WmiObject], ManagementException
+ FullyQualifiedErrorId : GetWMIManagementException,Microsoft.PowerShell.Commands.GetWmiObjectCommand

The OS is W7, .NET is 4.0,,,, I cant't find anything about it.

Any idea? Thanks!!