```typescript
const MODULUS = 1000000007;

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

  let a = 0, b = 1;
  for (let i = 0; i < n; i++) {
    [b, a] = [(a + b) % MODULUS, b];
  }

  return a;
}
```