```typescript

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 result = 0;
  let prev1 = 0, prev2 = 1;

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

  return result;
}
```