Snappy PDF runtime exception error with exit code 1

Please help me to figure out what could be the issue with the following function.

    public function generateProformaPdf()
   {
    $bookings = Booking::all();
    $date = Carbon::now()->toDateString();
    foreach($bookings as $booking){
        $filename = $booking->id.'--'.$date.".pdf";
        $invoice = SnappyPdf::loadview('pdf.proforma_invoice', compact('booking'));
        $invoice->save(storage_path("app/public/invoices/proformainvoices/", $filename));
        $proformaInvoice = new ProformaInvoice();
        $proformaInvoice->file = $filename;
        $proformaInvoice->save();
    }
}

The above function generates following RunTimeException

he exit status code '1' says something went wrong: stderr: "Loading 
pages (1/6) [> ] 0% [======> ] 10% [==============================> ] 
50% [============================================================] 100% 
QPainter::begin(): Returned false Error: Unable to write to destination 
Exit with code 1, due to unknown error. " stdout: "" command: 
/usr/local/bin/wkhtmltopdf --lowquality 
'/var/tmp/knp_snappy5bbdf5bc3f1978.38608827.html'
'/storage/app/public/invoices/proformainvoices/'.

All the other functions using SnappyPdf work ok.

via Chebli Mohamed