```
const MODULUS = 1000000007;

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

  if (n === 0) return 0;
  if (n === 1) return 1;

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