Set-SPUser : The parameterless Read method can only be used when this instance was initialized with an SPUser object.

I'm new to scripting and have run into an issue.  What we are trying to accomplish is for all our of SharePoint sites listed in a file, we want to gather a list of Users, compare them to the Admin array, then if they do not exist in the array, clear their permissions and add them back as Read only.  

$ListofSites =  Get-Content -Path "C:\sites.txt" 
$AdminUserLogin = @("DOMAIN\USER", "DOMAIN\USER2", 
                        )
foreach ($item in $ListofSites) 
    {
    Write-Host $item
    $Users = Get-SPUser -Web $item -Limit ALL | Format-Table -AutoSize  
        foreach ($user in $Users)
            {
            Write-Host $user
            If ($user -ne $AdminUserLogin) {Set-SPUser -Identity $._UserLogin -ClearPermissions -AddPermissionLevel 'Read'}
            Else {"User is an admin"}
            }
     }

I am getting this error--seems to be a piping error but I'm not really sure!  Help please!

Set-SPUser : The parameterless Read method can only be used when this instance was initialized with an SPUser object.
At C:\Scripts\SPReadOnly2.ps1:30 char:55
+             If ($user -ne $AdminUserLogin) {Set-SPUser <<<<  -Identity $._UserLogin -ClearPermissions -AddPermissionLevel 'Read'}
    + CategoryInfo          : InvalidData: (Microsoft.Share...SPCmdletSetUser:SPCmdletSetUser) [Set-SPUser], InvalidOperationException
    + FullyQualifiedErrorId : Microsoft.SharePoint.PowerShell.SPCmdletSetUser