```typescript
const MODULUS = 1000000007;

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

  let prev = 0;
  let curr = 1;

  for (let i = 2; i <= n; i++) {
    [prev, curr] = [curr % MODULUS, (prev + curr) % MODULUS];
  }

  return curr;
}
```