Find the index of a number in an array of Fibonacci numbers. Return error if number is not a Fibonacci number

Fibonacci.js

// Find the index of a number in an array of Fibonacci numbers.
// Return error if number is not a Fibonacci number.

function fibonacci(num) {

    var i,
        fib = []; // iterator, array

    // first two places in the array are always set
    fib[0] = 0;
    fib[1] = 1;

    num = parseInt(num, 10); // Make sure the number is an integer.

    if (isNaN(num)) {
        throw new Error("Must be an integer."); // Throw an error if it's not
    }

    // loop through from the third element to the number
    // passed to the function
    for (i = 2; i <= num; i+=1) {
        fib[i] = fib[i-2] + fib[i-1]; // generate the rest of the fibonacci array
        if (fib.indexOf(num) !== -1) { // if we find the number
            return fib.indexOf(num); //return it
        } else if (fib[i] > num) { // Suck. Number isn't a Fibonacci.
            throw new Error("Not a fibonacci number"); // Throw an error.
        }
    }

}