title: "Not a map for `map-get`" error after adding new elements to a mapping
body: <p>How can I merge a mapping into an existing mapping in sass?</p>
  
  <p>I am currently using sass maps in a <code>_config</code> partial to define my breakpoints, for example:</p>
  
  <pre><code>$breakpoints: (
      small: 35rem,
      medium: 55rem,
      large: 75rem,
      xlarge: 90rem,
      element-breakpoint-1: 100rem,
      element-breakpoint-2: 110rem 
  );
  
  @mixin breakpoint($width) {
      @media screen and (min-width: map-get($breakpoints, $width)) {
          @content;
      }
  }
  
  .element {
      width: 100px;
      @include breakpoint(element-breakpoint-1) {
          width: 200px;
      }
      @include breakpoint(element-breakpoint-2) {
          width: 300px;
      }
  }
  </code></pre>
  
  <p>Ideally I would like to be able to add new breakpoints to the existing mapping:</p>
  
  <pre><code>@function array-append($list, $value) {
      @return join($list, $value);
  }
  
  $breakpoints: array-append($breakpoints, (element-breakpoint-1: 100rem, element-breakpoint-1: 110rem));
  </code></pre>
  
  <p>The issue is that the breakpoint mixin does not see the new values in the <code>$breakpoints</code> list and SASS throws the following error:</p>
  
  <blockquote>
    <p>Error: $map: (("small" 35rem), ("medium" 55rem), ("large" 75rem), ("xlarge" 90rem), ("element-breakpoint-1" 100rem)) is not a map for `map-get'</p>
  </blockquote>