Data binding issue

When I try to compile this very short snippet, I get a warning that says: "Data binding will not be able to detect assignments to "historyPosition". Here is the code snippet:

<?xml version="1.0" encoding="utf-8"?>
<mx:WindowedApplication xmlns:mx="http://www.adobe.com/2006/mxml">
    <mx:HBox>
        <mx:Button id="btnBack" label="Back" click="html.historyBack()" enabled="{html.historyPosition > 0}"/>
        <mx:Button label="Forward" click="html.historyBack()" />
    </mx:HBox>
    <mx:HTML id="html" location="http://www.amazon.com" width="100%" height="80%" />
</mx:WindowedApplication>

I am merely using the html component's historyPosition value to make the back button enabled or disabled. Can anyone please tell me how I can do that? Thanks.