```ts
const MODULUS = 1000000007;

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

  if (n === 0) {
    return 0;
  }
  
  const prev = (n - 1 + MODULUS) % MODULUS;
  const curr = (n - 2 + MODULUS) % MODULUS;
  
  for (let i = 2; i <= n; i++) {
    prev = curr;
    curr = (prev + curr) % MODULUS;
  }
  
  return curr;
}
```