```
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");
  }

  let prev = 0; // fibMod(0)
  let current = 1; // fibMod(1)

  for (let i = 2; i <= n; i++) {
    const next = (prev + current) % MODULUS;
    [prev, current] = [current, next];
  }

  return current;
}
```