IHTMLSelectionObject.createRange() throws UnauthorizedAccessException

Question

I wrote the following code for retrieving the selected text from the current webpage:

IHTMLDocument2 mainDoc = ...
for ( int i = 0; i < mainDoc.frames.length; i++ ) {
    object refIndex = i;
    var frame = (IHTMLWindow2)mainDoc.frames.item( ref refIndex );
    IHTMLDocument2 frameDoc;
    try {
     frameDoc = frame.document;
    } catch ( UnauthorizedAccessException ex ) {
     // Source: http://codecentrix.blogspot.com/2008/02/when-ihtmlwindow2document-throws.html
     var sp = (IServiceProvider)frame;

     // Use IServiceProvider.QueryService to get IWebBrowser2 object.
     object brws = null;
     sp.QueryService( ref IID_IWebBrowserApp, ref IID_IWebBrowser2, out brws );

     // Get the document from IWebBrowser2.
     IWebBrowser2 browser = (IWebBrowser2)brws;
     frameDoc = (IHTMLDocument2)browser.Document;
    }
    var range = frameDoc.selection.createRange() as IHTMLTxtRange;
    if ( !string.IsNullOrEmpty(range.text) ) return range.text;
}
return string.Empty;

But on certain webpages, the call to frameDoc.selection.createRange() throws an UnauthorizedAccessException:

System.UnauthorizedAccessException was unhandled
  Message="Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED))"
  Source="Microsoft.mshtml"
  StackTrace:
       at mshtml.IHTMLSelectionObject.createRange()

How can I avoid this error?