Skriptproblem - Exchange 2013

# Exchange-Shell laden


$s = Get-PSSession | Where-Object {$_.ConfigurationName -eq 'Microsoft.Exchange'}  
if($s){
    Write-Host "Exchange Management Shell already loaded"  
}
else
{
    Write-Host "Exchange Management Shell not found - Loading..." -ForegroundColor Yellow  
 
        . "$env:ExchangeInstallPath\bin\RemoteExchange.ps1"  
 
        Write-Host "Exchange Management Shell loaded $nl"   -ForegroundColor green  
 
        Write-Host "Connecting to Exchange server $nl"  
 
        Connect-ExchangeServer -auto
 
        Write-Host "Connected to Exchange Server $nl" -ForegroundColor green  
}


# Postfach1


$mailbox = "buchhaltung@example.com"  
$group = "Buchhaltung"  

$mailboxFullRightUsers = Get-MailboxPermission $mailbox | ?{$_.AccessRights -eq "FullAccess" -and $_.IsInherited -eq $false} | %{$_.User.toString().split("\")[1]}  
$groupMemberUsers = Get-AdGroupMember -Identity $group | ?{$_.ObjectClass -eq "user"} | %{$_.SamAccountName}  

# SendOnBehalf Recht vergeben
Set-Mailbox $mailbox -GrantSendOnBehalfTo $groupMemberUsers

# Füge Gruppen-Member zur Shared Mailbox hinzu die noch keinen Vollzugriff haben
Compare $mailboxFullRightUsers $groupMemberUsers -PassThru | ?{$_.SideIndicator -eq "=>"} | %{echo "Füge User '$_' der Mailbox hinzu"; Add-MailboxPermission $mailbox -User $_ -AccessRights FullAccess -InheritanceType All | out-null}  

# Entferne Permissions wenn User nicht mehr Mitglied der Gruppe
Compare $mailboxFullRightUsers $groupMemberUsers -PassThru | ?{$_.SideIndicator -eq "<="} | %{echo "Entferne User '$_' aus der Shared Mailbox"; Remove-MailboxPermission $mailbox -User $_ -AccessRights FullAccess -Confirm:$false | out-null}  

Compare-Object : Das Argument kann nicht an den Parameter "ReferenceObject" gebunden werden, da es NULL ist.  
In C:\Skripte\Postfachberechtigungen.ps1:56 Zeichen:9
+ Compare $mailboxFullRightUsers $groupMemberUsers -PassThru | ?{$_.SideIndicator  ...
+         ~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [Compare-Object], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationErrorNullNotAllowed,Microsoft.PowerShell.Commands.CompareObjectCommand