A fairly Simple Script, but cannot see what is wrong here - powershell

Question

I have written a fairly simple script, to simply retrieve users via the UserPrinsipalName. It fails with the following error:
Get-AdUser : Property: 'UserPrincipalName' not found in object of type: 'System.Management.Automation.PSCustomObject'.
At C:\Temp\changeUPN.ps1:8 char:16
+ ... serObject = Get-AdUser -Filter {UserPrincipalName -eq $Obj.UserPrinci ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidArgument: (:) [Get-ADUser], ArgumentException
+ FullyQualifiedErrorId : ActiveDirectoryCmdlet:System.ArgumentException,Microsoft.ActiveDirectory.Management.Commands.GetADUser
$Csv = Read-Host "Enter Path to the Csv file"
$sps = Import-csv -Path $Csv
Foreach ($Obj in $Sps) {
Write-Host "Splitting UPN $($Obj.UserPrincipalName) to retreive domain information" -ForegroundColor Green
$Domain = ($Obj.UserPrincipalName -split "#")[1]
Write-Host "$($Obj.UserPrincipalName) is located in $($Domain)" -ForegroundColor Green
Write-Host "Getting AD Object" -ForegroundColor Green
$UserObject = Get-AdUser -Filter {UserPrincipalName -eq "$Obj.UserPrincipalName"} -Server $Domain
If ($UserObject) {
Write-Host "UserObject $($UserObject.DistinguishedName) Retrieved" -ForegroundColor Green
Write-Host "Changing UPN" -ForegroundColor Green
$NewUpn = $Obj.UserPrincipalName -Replace ("$Domain", "Contoso.com")
Write-Host "New UserPrincipalName = $($NewUpn)" -ForegroundColor Green
$UserObject | Set-AdUser -UserPrincipalName $NewUpn
}
Else {
Write-Host "Could not retrieve $($Obj.UserPrincipalName) from $($Domain)" -ForegroundColor Yellow
}
}