Getting "You cannot call a method on a null-valued expression." error during following script runs

[Net.ServicePointManager]::ServerCertificateValidationCallback={$true} 

$Dir = "C:\temp" 

foreach($item in (dir $dir)) 

{ 

    write-output "————————————–" 

    $fileName = $item.FullName 

    write-output $fileName 

    $ftp = [System.Net.FtpWebRequest]::Create("ftp://temp.com\temp\"+$item.Name) 

    $ftp = [System.Net.FtpWebRequest]$ftp 

    $ftp.UsePassive = $true 

    $ftp.UseBinary = $true 

    $ftp.EnableSsl = $true 

    $ftp.Credentials = new-object System.Net.NetworkCredential("xxx","xxx")

    $ftp.Method = [System.Net.WebRequestMethods+Ftp]::UploadFile 

    $rs = $ftp.GetRequestStream();    

    $reader = New-Object System.IO.FileStream ($fileName, [IO.FileMode]::Open, [IO.FileAccess]::Read, [IO.FileShare]::Read) 

    [byte[]]$buffer = new-object byte[] 4096

    [int]$count = 0

    do 

    { 

        $count = $reader.Read($buffer,0,$buffer.Length)

        $rs.Write($buffer,0,$count) 

    } while ($count -gt 0) 

    $reader.Close() 

    $rs.Close() 

    write-output "+transfer completed" 

  #  $item.Delete() 

   # write-output "+file deleted" 

}

I am getting follwing error -:

+         $rs.Write <<<< ($buffer,0,$count) 

    + CategoryInfo          : InvalidOperation: (Write:String) [], RuntimeException

    + FullyQualifiedErrorId : InvokeMethodOnNull

You cannot call a method on a null-valued expression.

i am unable to correct this error.by using this script i was able to upload files on FTP but suddenly it give this type of error can anyone rectify or provide me a better solution for uploading large size file on ftp using ps script.

thanks in advance