```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('n must be a non-negative integer');
  }

  let [a, b] = [0, 1];
  for (let i = 2; i <= n; i++) {
    const next = (a + b) % MODULUS;
    a = b;
    b = next;
  }
  return b;
}
```