无法将参数绑定到参数“Path”因为它是空的。参数似乎根本不为空

Question

检查远程计算机上是否存在文件：

 #Check if Dir exists on the machine we are copying from
            $dir = ($APPROD001Root + $instance + "\" + $type + "\" + $year + "\" + $monthFormatted)
            Write-Host $dir
            if(Test-Path -Path $dir){
                Write-Host ($dir + " exists!") 
                $files = Get-ChildItem -Path $dir

                #Check if the Dir has files in it
                if($files.Length -gt 0){
                    #Check if the folder exists on the machine we are copying towards, if not create it!
                    $dirReceivingMachine = ($DBStag01Root + $instance + "\" + $type + "\" + $year + "\" + $monthFormatted)
                    Write-Host $dirReceivingMachine
                    if($dirReceivingMachine -eq $null){
                        Write-Host "the path is null..."
                    }
                    $folderExists = Invoke-Command -ScriptBlock { Test-Path -Path $dirReceivingMachine } -Session $session 
                    if(!$folderExists){
                        Write-Host ("folder " + $dirReceivingMachine + " does not yet exist we want to create it!")
                        #Invoke-Command - ScriptBlock { New-Item -Path $dirReceivingMachine -ItemType Directory } -Session $session
                    }

设置$ folderexists始终投掷的命令命令

无法将参数与参数“路径”结合，因为它为null。 + categoryInfo：InvalidData：{:) [test-Path]，parameterBindingValidationException + firmomqualifyErrid：参数argumentValidationErrornullnotlower，Microsoft.powershell.commands.testpathcommand

我在终端中获得的输出IS

d：mydata \ 2018 \ 09 D：\ mydata \ 2018 \ 09存在！ D：\ mydata \ 2018 \ 09不能将参数绑定到 参数“路径”，因为它为空。 + categoryInfo：InvalidData：（:) [test-Path]，parameterBindingValidationException

As you can see the Write-Host "The path is null.." never fires, so the variable I am passing into Test-Path is not null yet PS says是在调用命令的时候...

不知道为什么要这样做