```ts
const MODULUS = 1000000007;

/**
 * The nth Fibonacci number, modulo 1000000007.
 */
export function fibMod(n: number): number {
  if (!Number.isInteger(n) || n < 0) {
    throw new Error('Input must be a non-negative integer.');
  }
  let ans1 = 0;
  let ans2 = 1;

  for (let _ = 0; _ < n; _++) {
    [ans1, ans2] = [ans2, (ans1 + ans2) % MODULUS];
  }

  return ans1;
}
```