```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('Input must be a non-negative integer');
  }

  if (n === 0) return 0;
  let first = 0, second = 1;

  for (let i = 2; i <= n; ++i) {
    const next = (first + second) % MODULUS;
    first = second;
    second = next;
  }

  return second;
}
```