```ts
const MODULUS = 1000000007;

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

  let prev = 0;
  let current = 1;

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

  return current;
}
```