Receiving odd error message in PS script - powershell

Question

I have a PowerShell script that goes out grabs the currently logged in user on a remote computer and gets the total size of their local profile C:\users\USERNAME\documents, pictures, music, etc. I have confirmed that the remote computer is online and the user is logged in. While the script is working, in the sense that it gets the folder size, it returns this message:
Get-WmiObject : Invalid parameter
At C:\Users\administrator\Desktop\get_folder_size.ps1:4 char:9
+ $user = Get-WmiObject Win32_ComputerSystem -ComputerName $system | Se ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (:) [Get-WmiObject], ManagementException
+ FullyQualifiedErrorId : GetWMIManagementException,Microsoft.PowerShell.Commands.GetWmiObjectCommand
You cannot call a method on a null-valued expression.
At C:\Users\administrator\Desktop\get_folder_size.ps1:5 char:1
+ $pos = $user.IndexOf("\")
+ ~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull
You cannot call a method on a null-valued expression.
At C:\Users\administrator\Desktop\get_folder_size.ps1:6 char:1
+ $user = $user.Substring($pos+1)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (:) [], RuntimeException
+ FullyQualifiedErrorId : InvokeMethodOnNull
I'm confused because if I'm reading the message correctly it shouldn't be able to grab the username thus not get the correct folder and the size.
Here is the code that I'm running
foreach ($system in Get-Content "C:\net_view.txt")
{
$colItems = 0
$user = Get-WmiObject Win32_ComputerSystem -ComputerName $system |
Select-Object -ExpandProperty UserName
$pos = $user.IndexOf("\")
$user = $user.Substring($pos+1)
$path = $system +"\c$\users\" + $user
$colItems = (Get-ChildItem $path -Recurse -ErrorAction SilentlyContinue |
Measure-Object -property length -sum)
"{0:N2}" -f ($colItems.sum / 1MB) + " MB"
Add-Content C:\users\administrator\desktop\profile_size.txt ($colItems.sum / 1MB)
Add-Content C:\users\administrator\desktop\profile_size.txt $system
}