title: Error mapping US Zip codes with topojson
body: <p>After loading a US zipcode topojson file I am getting an error in d3.js. <code>groupdata</code> is undefined on this line:</p>
  
  <pre><code>  function bind(group, groupData) {
       var i, n = group.length, m = groupData.length, n0 = Math.min(n, m), updateNodes = new Array(m), enterNodes = new Array(m), exitNodes = new Array(n), node, nodeData;
  </code></pre>
  
  <p>with an error of:</p>
  
  <pre><code>Uncaught TypeError: Cannot read property 'length' of undefined 
  </code></pre>
  
  <p>My code that is calling and creating the paths is:</p>
  
  <pre><code>  d3.json("data/us-atlas/us-zipcodes.json", function(error, topology) {
    svg.selectAll("path")
        .data(topojson.feature(topology, topology.objects.zipcodes).features)
        .enter()
        .append("path")
        .attr("d", path)
    });
  </code></pre>
  
  <p>I generated the zipcode topojson file from this repo: <a href="https://github.com/mbostock/us-atlas" rel="noreferrer">https://github.com/mbostock/us-atlas</a>. When I inspect the topology object on load I see 32893 arcs under topology.objects.zipcodes.</p>
  
  <p>I have successfully loaded counties under with the chloropleth example <a href="http://bl.ocks.org/mbostock/4060606" rel="noreferrer">http://bl.ocks.org/mbostock/4060606</a> and am using a similar pattern as that.</p>
  
  <p>I'm using d3.js version 3.2.8 and topojson.js version 1.2.3.</p>
  
  <p>Any ideas? Is it a bad zipcode file or am I calling it wrong?</p>