```
const MODULUS = 1000000007;

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

  if (n < 2) {
    return n;
  }

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