I implemented the Coda Slider tutorial successfully that is located here: http://jqueryfordesigners.com/coda-slider-effect/

The slider works great but I am getting a javascript error that I am not sure how to fix. The error says:

'0.offsetWidth' is null or not an object
coda-slider.js, line 19 character 3

Not sure how to fix it. Anyone have any ideas? Here is my js and css (don't think I need to upload the HTML but let me know if that helps).

JS (coda-slider.js)

// when the DOM is ready...
$(document).ready(function () {

var $panels = $('#slider .scrollContainer > div');
var $container = $('#slider .scrollContainer');

// if false, we'll float all the panels left and fix the width 
// of the container
var horizontal = true;

// float the panels left if we're going horizontal
if (horizontal) {
  $panels.css({
    'float' : 'left',
    'position' : 'relative' // IE fix to ensure overflow is hidden
  });

  // calculate a new width for the container (so it holds all panels)
  $container.css('width', $panels[0].offsetWidth * $panels.length); <------line 19
}

// collect the scroll object, at the same time apply the hidden overflow
// to remove the default scrollbars that will appear
var $scroll = $('#slider .scroll').css('overflow', 'hidden');

// apply our left + right buttons
$scroll
  .before('<img class="scrollButtons left" src="/images/layout/navigation/scroll_left.png" />')
  .after('<img class="scrollButtons right" src="/images/layout/navigation/scroll_right.png" />');

// handle nav selection
function selectNav() {
  $(this)
    .parents('ul:first')
      .find('a')
        .removeClass('selected')
      .end()
    .end()
    .addClass('selected');
}

$('#slider .navigation').find('a').click(selectNav);

// go find the navigation link that has this target and select the nav
function trigger(data) {
  var el = $('#slider .navigation').find('a[href$="' + data.id + '"]').get(0);
  selectNav.call(el);
}

if (window.location.hash) {
  trigger({ id : window.location.hash.substr(1) });
} else {
  $('ul.navigation a:first').click();
}

// offset is used to move to *exactly* the right place, since I'm using
// padding on my example, I need to subtract the amount of padding to
// the offset.  Try removing this to get a good idea of the effect
var offset = parseInt((horizontal ? 
  $container.css('paddingTop') : 
  $container.css('paddingLeft')) 
  || 0) * -1;


var scrollOptions = {
  target: $scroll, // the element that has the overflow

  // can be a selector which will be relative to the target
  items: $panels,

  navigation: '.navigation a',

  // selectors are NOT relative to document, i.e. make sure they're unique
  prev: 'img.left', 
  next: 'img.right',

  // allow the scroll effect to run both directions
  axis: 'xy',

  onAfter: trigger, // our final callback

  offset: offset,

  // duration of the sliding effect
  duration: 500,

  // easing - can be used with the easing plugin: 
  // http://gsgd.co.uk/sandbox/jquery/easing/
  easing: 'swing'
};

// apply serialScroll to the slider - we chose this plugin because it 
// supports// the indexed next and previous scroll along with hooking 
// in to our navigation.
$('#slider').serialScroll(scrollOptions);

// now apply localScroll to hook any other arbitrary links to trigger 
// the effect
$.localScroll(scrollOptions);

// finally, if the URL has a hash, move the slider in to position, 
// setting the duration to 1 because I don't want it to scroll in the
// very first page load.  We don't always need this, but it ensures
// the positioning is absolutely spot on when the pages loads.
scrollOptions.duration = 1;
$.localScroll.hash(scrollOptions);

});

CSS

#slider {
  margin-left: 35px;
  position: relative;
  width: 875px;

}

.scroll {
  position: relative;
  width: 875px;
  height: 268px;
  overflow: auto; /* fix for IE to respect overflow */
  background: #FFFFFF scroll 0;
}

.scrollContainer div.panel {
  position: relative;
  height: 210px;
  width: 875px; /* change to 560px if not using JS to remove rh.scroll */
}

.scrollButtons {
  position: absolute;
  top: 115px;
  cursor: pointer;
}

.scrollButtons.left {
  left: -20px;
}

.scrollButtons.right {
  right: -20px;
}