找出Invoice谓词里的bug

type Invoice = { id: string; total: number };

function isInvoice(value: unknown): value is Invoice {
  return (
    typeof value === "object" &&
    value !== null &&
    "id" in value
  );
}

function formatInvoice(value: unknown): string {
  if (!isInvoice(value)) return "invalid";
  return `${value.id}:${value.total.toFixed(2)}`;
}