Problem with IFRAME

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ASP.NET

I had the problem with the issue how to stretch IFRAME to 100% height depending on the content size in IFRAME. I found out 1 trick here. And all has been working properly in the test page (almost empty page whith this iframe only). But when I inserted this code into page with MasterPage I got an error: Microsoft JScript runtime error: 'document.frames[...].document' is null or not an object. What to do next? How to fix this problem?

The suggested solution I found out earlier here:

<script type="text/javascript">
        function getElement(aID) {
            return (document.getElementById) ?
            document.getElementById(aID) :

document.all[aID];
        }

        function getIFrameDocument(aID) {
            var rv = null;
            var frame = getElement(aID);
            // if contentDocument exists, W3C 

            //compliant(e.g.Mozilla)
            if (frame.contentDocument)
                rv = frame.contentDocument;
            else // bad Internet Explorer  ;)
                rv = document.frames[aID].document;
            return rv;
        }

        function adjustMyFrameHeight() {
            var frame = getElement("frmLection");
            var frameDoc = getIFrameDocument("frmLection");
            frame.height = frameDoc.body.offsetHeight;
        }
    </script>

and HTML content:

<iframe id="frmLection" frameborder="0" width="100%" scrolling="no">
                        src="Lection1.html" runat="server">
                    </iframe>

Could somebody give me some advice? Thanks in advance.