试图获取非对象的属性-Laravel

我现在要做的是在“付款”表和“发票”表之间建立多对多关系……但是我的问题是，当我尝试添加新付款并选择发票列表时，laravel给了我“试图在第217行获取非对象的属性。

这是我的代码：

$ selectedInvoices = $ input ['invoice'];

    $invoices = collect([]);
    foreach ($selectedInvoices as $invoice) {
        $invoices = $invoices->merge([Invoice::find(Invoice::getPrivateId($invoice))]);
    }
    $invoices->all();

    $invoices = $invoices->sortByDesc(function($invoice)
    {
        return $invoice->invoice_date;
    });
    $invoices->values()->all();

    $totalAmount = 0;

    foreach ($invoices as $invoice)
    {
        $totalAmount = $totalAmount + $invoice->balance;
    }

    if($payment->amount > $totalAmount){
        foreach ($invoices as $invoice){
            $adjustment = $invoice->balance * -1;
            $partial = max(0, $invoice->partial - 0);
            $invoice->updateBalances($adjustment, $partial);
            $invoice->updatePaidStatus();
            $payment->invoices()->attach($invoice->id) ;
        }
        $credit = Credit::createNew();
        $credit->client_id = $clientId;
        $credit->amount = $payment->amount - $totalAmount ;
        $credit->balance = $payment->amount - $totalAmount ;
        $credit->credit_date = $payment->payment_date;
        $credit->private_notes = 'Crédit créé automatiquement suite au payement '.$payment->id.
        ' avec un montant de '.$payment->amount.' à la date de '.$payment->payment_date;
        $credit->save();
    }

    if($payment->amount <= $totalAmount)
    {
         /*line 217*/   while ($payment->amount >= $invoices->last()->balance){
            $payment->amount = $payment->amount - $invoices->last()->balance ;

            $adjustment = $invoices->last()->balance * -1;
            $partial = max(0, $invoices->last()->partial - 0);
            $invoices->last()->updateBalances($adjustment, $partial);
            $invoices->last()->updatePaidStatus();

            $payment->invoices()->attach($invoices->last()->id) ;

            $invoices->pop();
            $invoices->all();
            }

        if ($payment->amount < $invoices->last()->balance){
            $adjustment = $payment->amount * -1;
            $partial = max(0, $invoices->last()->partial - $payment->amount);

            $invoices->last()->updateBalances($adjustment, $partial);
            $invoices->last()->updatePaidStatus();
            $payment->invoices()->attach($invoices->last()->id) ;
        }
    }

ps：当我给出金额>我的发票总额时，当我给出金额<我的发票总额时，它也起作用..