```ts
const MODULUS = 1000000007;

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

    let a = 0;
    let b = 1;

    for (let i = 2; i <= n; i++) {
        const temp = a + b;
        a = temp % MODULUS;
        b = temp - a % MODULUS;
    }

    return a;
}
```