I do not understand why my variable assignment: `var mn = $("nav");` is not working in this piece of code:

```
var mn = $("nav");
var randomNum = 23;

$(window).scroll(function() {
  if( $(this).scrollTop() > 400 ) {
    $("nav").addClass("main-nav-scroll");
    alert(randomNum);
  } else {
    mn.removeClass("main-nav-scroll");
  }
});

```

When I manually write it out in `$("nav").addClass(...);` it works perfectly. I thought the problem was maybe the scope of the variable so I added the `randomNum` variable to print out and it does so just fine. I'm really stumped. It took me forever to find this simple error So I'd like to understand for next time. Thanks.