Custom PowerShell Object Property Issue

I'm attempting to create a custom PowerShell object using data from the WebAdministration module for IIS7. The object should capture the ClientIPAddress and SiteID from the GetExecuteRequest WmiMethod for Worker Processes in IIS.

$p = Get-WmiObject WorkerProcess -Namespace root\WebAdministration -ComputerName . | Invoke-WmiMethod -Name GetExecutingRequests | Select-Object -ExpandProperty OutputElement
$wpob = @()

foreach($w in $p)
{
            $webrequests = New-Object PSObject
            $webrequests | Add-Member -Type NoteProperty -Name ClientIP -Value ($w | select -ExpandProperty ClientIPAddress)
            $webrequests | Add-Member -Type NoteProperty -Name SiteID -Value ($W | select -ExpandProperty SiteID)
            $wpobj += $webrequests
        }

The Problem:

Object ($wpobj) contains the 'ClientIP' and 'SiteID' under get-member, but you cannot get those values by using $wpobj.clientip or $wpobj.siteid. Only way I could retrieve those values were to use $wpobj | select clientip or $wpobj | select siteid

Not being able to get the values for the properties in the above mentioned way makes it more difficult to sort/group the data as I can't select the values later on down the pipeline.

Anyone know what I may be missing? I have not seen this before. Only in this particular case

Solution was to upgrade Powershell to V4.0. IE.invoke won't work on earlier versions of Powershell.