Weird PowerShell error

Got a weird issue with a PowerShell tool in my PDQ server. The script is as follows

  $computer = '$(Computer:TARGET)'  
  $host.ui.RawUI.WindowTitle = "Logon details of $Computer"
  $ErrorActionPreference = 'SilentlyContinue'
  $users = $false
  $locked = $false
  $users = (Get-WmiObject -Class win32_computersystem -ComputerName 
  $computer).username.Split("\")[1]
  If ((Get-Process logonui -ComputerName $Computer) -and ($users))
                {
                $Locked = $True
                }
 foreach ($user in $users) {
    $Output = New-Object PSObject
    $Output | Add-Member noteproperty Computer $Computer
    $Output | Add-Member noteproperty Username $User
    $Output | Add-Member noteproperty Locked $Locked
    $Output
}

Running this, I get the following error message:

You cannot call a method on a null-valued expression.
At C:\ProgramData\Admin
Arsenal\Temp\PDQInventoryConsole\9240\0c70b1e4-c5ba-4021-8b87-30fda7f784b7\97029f86-3b48-402e-b9ec-bdad13666cd5.ps1:7
char:1
+ $users = (Get-WmiObject -Class win32_computersystem -ComputerName $co ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo          : InvalidOperation: (:) [], ParentContainsErrorRecordException
+ FullyQualifiedErrorId : InvokeMethodOnNull

If I change the first line of the script from

$computer = '$(Computer:TARGET)'

To

$computer = Read-Host 'Enter computer name'

And then run it from ISE, I get the desired output

Computer    Username Locked
--------    -------- ------
spare-01    False    False

The server has been rebooted several times since. The issue was first noticed at some point during version 16, but this was part of a bigger issue, which I was told would be fixed in version 17 so didn't report this sooner. We've just upgraded to version 17, although the bigger issue that this was originally part of has been solved, this issue is still present.

Any suggestions on what I can do to resolve this issue?