```ts
const MODULUS = 1000000007;

/**
 * The nth Fibonacci number, modulo 1000000007.
 */
export function fibMod(n: number): number {
  if (!Number.isInteger(n) || n < 0) {
    throw new Error("n must be a non-negative integer");
  }

  let a = 0;
  let b = 1;

  for (let i = 2; i <= n; i++) {
    const temp = (a + b) % MODULUS;
    a = b;
    b = temp;
  }

  return a;
}
```