php - php soapclient での「オブジェクト参照がオブジェクトのインスタンスに設定されていません」というエラー

Question

このような投稿がいくつかあることは知っていますが、私の問題を解決する投稿は見たことがありません。請求書を挿入するために kashflow API を呼び出しています。InvoiceLine タイプのオブジェクトのコレクションを必要とするタイプ「Invoice」のオブジェクトが必要です。

ここを参照してください：

http://www.kashflow.com/developers/soap-api/invoice/

http://www.kashflow.com/developers/soap-api/InvoiceLine/

ここに私が現在使用しているコードがあります。

class Invoice{

    public $InvoiceDBID = ''; //TSystem-wide unique number for this invoice (used internally only)
    public $InvoiceNumber = ''; //Unique invoice number
    public $InvoiceDate = '';   //Invoice date
    public $DueDate = '';   //Invoice due date
    public $SuppressTotal = ''; //1 to suppress the total (only used on Quotes)
    public $ProjectID = ''; //The project ID for this invoice. 
    public $CurrencyCode = '';  //The currency code to be used for the invoice e.g. USD, GBP, ZAR
    public $ExchangeRate = '';  //The exchange rate for the currency used. 
    public $Paid = 0;   //1 for Paid, 0 for Unpaid. Set to 0 on Insert calls
    public $CustomerID = '';    //The ID of the customer (or supplier int he case of receipts)
    public $CustomerReference = ''; 
    public $EstimateCategory = '';  //The category of the quote (only relevant when with Quotes)
    public $NetAmount = 0;  //The total net amount of the invoice
    public $VatAmount = 0;  //The total VAT amount of the invoice
    public $AmountPaid = 0; //The sum of all payments made to this invoice
    public $Permalink = ''; //A permanent link to access a PDF of the invoice.
    public $Lines = ''; //A collection of type InvoiceLine.

}

class InvoiceLine{

    public $LineID = ''; //System-wide unique number for this line
    public $Quantity   = ''; //Quantity invoiced
    public $Description   = ''; //Description of item invoiced 
    public $Rate = '';  //Rate per item
    public $ChargeType = '';    //For Purchases: Related Nominal ID From GetNominalCodes.
    public $ProjID = '';    //The Project to which this line should be assigned. Leave as 0 if in doubt 
    public $VatAmount = ''; //The VAT amount for this line
    public $VatRate = '';   //The VAT rate applied to this line (e.g. 15 for 15%). For Vat Exempt (the n/a code) use -1
    public $Sort = '';  //Sort order value
    public $ProductID  = '';    //An integer value representing the ID of a SubProduct (from GetSubProducts). Set to 0 for “None”
    public $ValuesInCurrency = '';  //Set to “1? if the Rate and VATAmount is already in foreign currency

}

$client = new SoapClient("https://securedwebapp.com/api/service.asmx?WSDL");    

$invoice_line = new InvoiceLine();
$invoice_line->Quantity = 1;
$invoice_line->Description = 'Delicious Irish Whisky';
$invoice_line->Rate = 20;
$invoice_line->ChargeType = 5614031;
$invoice_line->VatAmount = 4.00;
$invoice_line->VatRate = 20;
$invoice_line->ProductID = 618736;

$invoice = new Invoice();
$invoice->InvoiceDate = '11/10/2012';
$invoice->DueDate = '08/11/2012';               
$invoice->CurrencyCode = 'GBP';
$invoice->ExchangeRate = 1.0000;
$invoice->CustomerID = '1780661';
$invoice->Lines = array($invoice_line);


$parameters['UserName'] = "xxxxxxxxx";
$parameters['Password'] = "xxxxxxxxx";
$parameters['Invoice'] = $invoice;
$response = $client->InsertInvoice($parameters);