```ts
const MODULUS = 1000000007;

/**
 * The nth Fibonacci number, modulo 1000000007.
 */
export function fibMod(n: number): number {
  if (typeof n !== 'number' || n < 0) {
    throw new Error('n must be a non-negative integer');
  }
  
  let a = 0;
  let b = 1;
  let result = 0;
  
  for (let i = 2; i <= n; i++) {
    const temp = (a + b) % MODULUS;
    a = b;
    b = temp;
    if (i === n) {
      result = b;
    }
  }
  
  return result;
}
```