```typescript
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 new Error('n must be a non-negative integer.');
  }
  if (n === 0) return 0;
  if (n === 1) return 1;

  let prev = 0, curr = 1;
  for (let i = 2; i <= n; i++) {
    [prev, curr] = [curr, (prev + curr) % MODULUS];
  }
  return curr;
}
```