```ts
export function fibMod(n: number): number {
  if (typeof n !== 'number' || !Number.isInteger(n) || n < 0) {
    throw new Error('n must be a non-negative integer');
  }

  let a = 0, b = 1;
  for (let i = 2; i <= n; i++) {
    const temp = a + b;
    a = b;
    b = temp % MODULUS;
  }
  return b;
}
```