```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.');
  }

  if (n <= 1) {
    return n;
  }

  let prev0 = 0, prev1 = 1;

  for (let i = 2; i <= n; i++) {
    const temp = (prev0 + prev1) % MODULUS;
    prev0 = prev1;
    prev1 = temp;
  }

  return prev1;
}
```