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

    let a = 0, b = 1;
    for (let i = 2; i <= n; i++) {
        const next = (a + b) % MODULUS;
        a = b;
        b = next;
    }
    return b;
}
```