```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('Input must be a non-negative integer');
  }
  let a = 0, b = 1;
  for (let i = 0; i < n; i++) {
      [a, b] = [b, (a + b) % MODULUS];
  }
  return a;
}
```