```ts
/** A net price with a whole-percent tax added, in whole pence. */
export function grossPrice(net: number, rate: number): number {
  if (net === null || net === undefined) {
    throw new Error("Input cannot be null or undefined");
  }
  if (rate < 0) {
    throw new Error("Rate cannot be negative");
  }

  const tax = Math.floor(net * rate / 100);
  return net + tax;
}
```