```typescript
const MODULUS = 1000000007;

/**
 * The nth Fibonacci number, modulo 1000000007.
 *
 * @param n - The index of the Fibonacci number to return (0-indexed).
 * @throws {Error} If n is not a non-negative integer.
 */
export function fibMod(n: number): number {
  if (typeof n !== 'number' || n < 0 || n % 1 !== 0) {
    throw new Error('n must be a non-negative integer');
  }

  let a = 0;
  let b = 1;

  for (let i = 2; i <= n; i++) {
    [a, b] = [b, (a + b) % MODULUS];
  }

  return a;
}
```