mapviewer fails on valid TopoJSON

Run this code:

  <div id="topojson-map" style="height:300px">
  <script>
    var map = g1.mapviewer({
      id: 'topojson-map',
      layers: {
        worldMap: { type: 'tile', url: 'http://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png' },
        usopojson: {
          type: 'topojson',
          url: 'us-10m.v1.json',
        }
      }
    })
  </script>

... with us-10m.v1.json

This raises the following error: