JavaScript runtime error: in IE only

Am getting an error when i run my site in IE (local host)

the error is as follows:

Unhandled exception at line 1440, column 9 in eval code

0x800a138f - JavaScript runtime error: Unable to get property 'top' of undefined or null reference

If there is a handler for this exception, the program may be safely continued.

This is the code it goes to:

 $window.scroll(function () {

if ($window.scrollTop() > offset.top) {

        $sidebar.stop().animate({
            marginTop: $window.scrollTop() - offset.top + topPadding
        });
    } else {
        $sidebar.stop().animate({
            marginTop: 0
        });
    }
});

the highlighted code is where it keeps hitting when i scroll on the page.

Am not sure why this is happening, has this happened to anyone? Or any ideas why its doing this.

Thanks

It's all in the error message:

"Unable to get property 'top' of undefined or null reference"

That means that whatever you're trying to get top from, is undefined or null.
Specifically: offset is.