```ts
const MODULUS = 1000000007;

export function fibMod(n: number): number {
  if (n < 0 || !Number.isInteger(n)) {
    throw new Error("n must be a non-negative integer");
  }

  let a = 0, b = 1;
  for (let i = 0; i < n; i++) {
    [a, b] = [b, (a + b) % MODULUS];
  }
  return a;
}
```