尝试通过PowerShell创建新工作项时，项目集合获取服务未返回任何值

我试图使用powershell和tfs-api创建一个新的工作项类型。

这是我试图运行的脚本，用于创建一个新的工作项类型

[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.Client")
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.Build.Client")  
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.Build.Common") 
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.WorkItemTracking.Client") 
[string] $tfsCollectionUrl = "http://r2-09-tfs:8080/tfs/DefaultCollection"
$teamProjectCollection = [Microsoft.TeamFoundation.Client.TfsTeamProjectCollectionFactory]::GetTeamProjectCollection($tfsCollectionUrl)
$ws = $teamProjectCollection.GetService([type]"Microsoft.TeamFoundation.WorkItemTracking.Client.WorkItemStore")
$proj = $ws.Projects["TestTeamProject"]
$wit = $proj.WorkItemTypes["Task"]
$workitem = $wit.NewWorkItem()
$workItem.Title = "Sample Task Title 2"
$workItem.Description = "Sample Description"
$workitem.AreaPath = "TestTeamProject"
$workitem.IterationPath = "TestTeamProject"
$workItem.Save()
Write-Host "The TFS work item number is: " $workItem.Id

当我运行这个脚本时，得到一个错误，上面写着Cannot index into a null array。在$proj=$ws。项目["测试团队项目"]

我的猜测是，workitemstore变量$ws为null，这一行中我遗漏了什么吗？

[void]System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.Client")
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.Build.Client")  
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.Build.Common") 
[void][System.Reflection.Assembly]::LoadWithPartialName("Microsoft.TeamFoundation.WorkItemTracking.Client") 
[string] $tfsCollectionUrl = "http://r2-09-tfs:8080/tfs/DefaultCollection"
$teamProjectCollection = [Microsoft.TeamFoundation.Client.TfsTeamProjectCollectionFactory]::GetTeamProjectCollection($tfsCollectionUrl)
[Microsoft.TeamFoundation.WorkItemTracking.Client.WorkItemStore]$ws = $teamProjectCollection
$proj = $ws.Projects["TestTeamProject"]
$wit = $proj.WorkItemTypes["Task"]
$workitem = $wit.NewWorkItem()
$workItem.Title = "Sample Task Title 2"
$workItem.Description = "Sample Description"
$workitem.AreaPath = "TestTeamProject"
$workitem.IterationPath = "TestTeamProject"
$workItem.Save()
Write-Host "The TFS work item number is: " $workItem.Id