style.backgroundImage will fire onpropertychange twice

We all know IE is weird:

<input id="mytestinput" onclick="this.style.backgroundImage='url(http://blog.youkuaiyun.com/images/authorship.gif)';" type="button" value="change background image" />
<script>
document.getElementById('mytestinput').attachEvent(
    'onpropertychange', function () { alert(event.propertyName); });
</script>

click the button will alert twice.

If u set backgroundImage to 'none', it will also alert twice, however, set to ''(empty string), will only once...