```ts
const MODULUS = 1000000007;

/**
 * The nth Fibonacci number, modulo 1000000007.
 */
export function fibMod(n: number): number {
  
  if (typeof n !== 'number') throw new Error('n must be a number');
  if (n < 0) throw new Error('n must be a non-negative integer');
  if (n % 1 !== 0) throw new Error('n must be an integer');
  
  let prev = 0;
  let curr = 1;
  for (let i = 2; i <= n; i++) {
    [prev, curr] = [curr, (prev + curr) % MODULUS];
  }
  return curr;
}
```