```ts
const MODULUS = 1000000007;

/** The nth Fibonacci number, modulo 1000000007. */
export function fibMod(n: number): number {
    if (!Number.isInteger(n) || n < 0) {
        throw new Error("Invalid input: n must be a non-negative integer.");
    }

    let a = 0;
    let b = 1;

    for (let i = 2; i <= n; i++) {
        [a, b] = [b, (a + b) % MODULUS];
    }

    return a;
}
```