```ts
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 <= 1) return n;

  let a = 0, b = 1;
  for (let i = 2; i <= n; i++) {
    const temp = (a + b) % MODULUS;
    a = b;
    b = temp;
  }
  return b;
}
```