title: jQuery returns not a number only when I use subtract
body: <p>I am having an issue with jQuery that I can't seem to figure out.</p>
  
  <p>If I do <code>console.log($(window).width()/2)</code> I get a numerical value.</p>
  
  <p>If I do <code>$('.modal_box').outerWidth()</code> I get a numerical value.</p>
  
  <p>However if I try to do <code>console.log($(window).width()/2 - $('.modal_box').outerWidth())</code> I get NaN. If I do the same equation but replace the <code>-</code> with <code>+</code> it works. </p>
  
  <p>The window width is always wider than the <code>.modal_box</code>.</p>
  
  <p>Why could this be? It is very frustrating.</p>