```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.");
  }

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

  let prev2 = 0; // fibMod(n-2)
  let prev1 = 1; // fibMod(n-1)
  for (let i = 2; i <= n; i++) {
    const current = (prev1 + prev2) % MODULUS;
    prev2 = prev1;
    prev1 = current;
  }
  return prev1;
}
```