title: Can't get susy to use gutter in 'px' while in math:static mode
body: <p>I want to create a fixed non-flexible very damn static grid of 1200px. 
  So I figured out, I need 90px for column-width, 12 columnts, 10px of gutter and 1200px of max-width. </p>
  
  <p>Well.. the following are my settings and it gives me an error "invalid null opearation: 11 times null "</p>
  
  <pre><code>$susy: (
    flow: ltr,
    math: static,
    output: float,
    gutter-position: after,
    container: auto,
    container-position: center,
    columns: 12,
    gutters: 10px,
    column-width: 90px,
    global-box-sizing: content-box,
    last-flow: to,
    debug: (
      image: show,
      color: rgba(#66f, .25),
      output: overlay,
      toggle: top right,
    ),
    use-custom: (
      background-image: true,
      background-options: false,
      box-sizing: true,
      clearfix: false,
      rem: true,
    )
  );
  </code></pre>
  
  <p>style.scss></p>
  
  <pre><code>    @import "grids";
  
  body{
      @include container($susy);
      max-width: 1200px;
  
      border: solid thin red;
      height:10px;
  
  }
  </code></pre>