Problems in reading a commandline json object using powershell

I have a powershell script "test.ps1" which takes a json string as a command line input & tries to parse it.

The powershell script is as below -

param(
 [string]$json = $(throw 'test')
)
$currentPath = Get-Location
[Reflection.Assembly]::LoadFile("$currentPath\Newtonsoft.Json.dll")
$result = [Newtonsoft.Json.Linq.JObject]::Parse($json)
foreach($unit in $result["DevResults"]) {Write-Host $unit.TechnologyName.ToString()}

But it is giving me the below error -

PS C:\Users\aghosh.RJFDEV\Documents> ./test.ps1 '{"DevResults":[{"TechnologyName":"TFS","RuleName":"Alt CI ID for ESB","OutputValue":"ESClientCenter"},{"TechnologyName":"TFS","RuleName":"TFS Team Project Name","OutputValue":"ClientCenter"}],"QaResults":[{"TechnologyName":"TFS","RuleName":"Alt CI ID for ESB","OutputValue":"ESClientCenter"},{"TechnologyName":"TFS","RuleName":"TFS Team Project Name","OutputValue":"ClientCenter"}],"PreProdResults":[{"TechnologyName":"TFS","RuleName":"Alt CI ID for ESB","OutputValue":"ESClientCenter"},{"TechnologyName":"TFS","RuleName":"TFS Team Project Name","OutputValue":"ClientCenter"}],"ProdResults":[{"TechnologyName":"TFS","RuleName":"Alt CI ID for ESB","OutputValue":"ESClientCenter"},{"TechnologyName":"TFS","RuleName":"TFS Team Project Name","OutputValue":"ClientCenter"},{"TechnologyName":"TFS","RuleName":"Process Template","OutputValue":"Raymond James CMMI V3.5"}]}'

GAC    Version        Location                                                                                                                                                                                                                                     
---    -------        --------                                                                                                                                                                                                                                     
False  v2.0.50727     C:\Users\aghosh.RJFDEV\documents\Newtonsoft.Json.dll                                                                                                                                                                                         
You cannot call a method on a null-valued expression.
At C:\Users\aghosh.RJFDEV\Documents\test.ps1:13 char:82
+ foreach($unit in $result["DevResults"]) {Write-Host $unit.TechnologyName.ToString <<<< ()}
    + CategoryInfo          : InvalidOperation: (ToString:String) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull

You cannot call a method on a null-valued expression.
At C:\Users\aghosh.RJFDEV\Documents\test.ps1:13 char:82
+ foreach($unit in $result["DevResults"]) {Write-Host $unit.TechnologyName.ToString <<<< ()}
    + CategoryInfo          : InvalidOperation: (ToString:String) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull

My powershell version is 2.0 & the Json.Net library version is 3.5.

Please help me with the error or please suggest any other suitable approach.

Thanking in advance.