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