```ts
const MODULUS = 1000000007;

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

  let a = 0;
  let b = 1;

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

  return a;
}
```