```ts
const MODULUS = 1000000007;

/** The nth Fibonacci number, modulo 1000000007. */
export function fibMod(n: number): number {
  if (n < 0 || !Number.isInteger(n)) {
    throw new Error("n must be a non-negative integer");
  }
  
  if (n === 0) {
    return 0;
  }
  
  let prev2 = 0;
  let prev1 = 1;
  
  for (let i = 2; i <= n; i++) {
    const current = (prev1 + prev2) % MODULUS;
    prev2 = prev1;
    prev1 = current;
  }
  
  return prev1;
}
```