getElementById from parent works for Firefox not  for IE

I am trying to access an element in my Edge Animate animation (which is a menu bar) from the parent document.  The element has an onClick event which is triggered depending on the #bookmark in the URL of the parent web page.  My code works perfectly in Firefox but does not work in Internet Explorer(10).  IE is unable to see any elements within the 'Stage' div whereas Firefox can.

This is the JavaScript code on my parent page: -

<script language='javascript'>

      var thisPage = window.location.pathname;

      var fullurl = document.URL;

      var xxx = fullurl.substring(fullurl.indexOf('#'));

      var pageString = xxx.replace("#", "");

      pageString = pageString.replace("http://www.mydomain.com/portfolio/photography.html", "");

      if (pageString == "corporate") {

            window.onload = function()  { 

            var iframe = document.getElementById('U10511_animation');

            var innerDoc = (iframe.contentDocument) ? iframe.contentDocument : iframe.contentWindow.document;

            var corporateRectangle = innerDoc.getElementById('Stage_Corporate_Rectangle');

            corporateRectangle.click();

      }

};

</script>

The above code will select the Corporate tab in the menu when viewed in Firefox but not IE when the URL has the suffix #corporate.

When I insert an 'alert' for the variable 'corporateRectangle' in Firefox it returns [HTMLObj] and in IE it returns 'null'.

Any ideas anyone?  Thanks.