```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");
  }

  if (n === 0) return 0;
  if (n === 1) return 1;

  let prev2 = 0;
  let prev1 = 1;
  let current = 0;

  for (let i = 2; i <= n; i++) {
    current = (prev1 + prev2) % MODULUS;
    prev2 = prev1;
    prev1 = current;
  }

  return current;
}
```