I am trying to rotate an image using javascript and I was trying some code I found on adobes website that does what I was looking for. When I run it it gives me the error: Uncaught TypeError: Cannot set property 'src' of null

It keeps repeating every 5 second interval which is the amount of time between each image

Here is the javascript:

 (function () {

        var rotator = document.getElementById('rotator');  // change to match image ID

        var imageDir = 'images/';                          // change to match images folder

        var delayInSeconds = 5;                            // set number of seconds delay

        // list image names

        var images = ['image2.jpg', 'image3.jpg', 'image4.jpg', 'image1.jpg'];



        // don't change below this line

        var num = 0;

        var changeImage = function () {

            var len = images.length;

            rotator.src = imageDir + images[num++];

            if (num == len) {

                num = 0;

            }

        };

        setInterval(changeImage, delayInSeconds * 1000);

    })();

and here is the html for the image:

<img src=images/image1.jpg alt=rotating image width=400 height=300 id=rotator />

It shows the image1.jpg, but does not rotate it. Is there something blatantly obvious that I am not seeing? Wouldn't surprise me!

Thanks to anyone that can offer advice.