Usar querySelector para localizar elementos aninhados dentro de um modelo Polymer retorna nulo

Estou tentando usar guias de papel dentro de um novo elemento (lista de guias), mas após imprimir guias, não consigo usar o querySelector para alterar o selecionado.

Código do elemento (sem estilo):

<link rel="import" href="../components/polymer/polymer.html">
<link rel="import" href="../sprint-service/sprint-service.html">
<link rel="import" href="../components/paper-tabs/paper-tabs.html">

<polymer-element name="tab-list" attributes="show">
  <template>
    <sprint-service id="service" sprints="{{sprints}}"></sprint-service>   
    <paper-tabs selected="all" valueattr="name" self-end>
      <paper-tab name="all">ALL</paper-tab>
      <template repeat="{{sprint in sprints}}">
        <paper-tab name="{{sprint.id}}">{{sprint.id}}</paper-tab>
      </template>
    </paper-tabs>
  </template>
  <script>
    Polymer('tab-list', {
      ready: function() {
        var tabs = document.querySelector('paper-tabs');
        tabs.addEventListener('core-select', function() {
          list.show = tabs.selected;
        }) 
      } 
    });
  </script>
</polymer-element>

Código Index.html (sem estilo):

<!doctype html>
<html>

<head>
  <title>unquote</title>
  <meta name="viewport" content="width=device-width, minimum-scale=1.0, initial-scale=1.0, user-scalable=yes">
  <script src="../components/platform-dev/platform.js"></script>
  <link rel="import" href="../components/font-roboto/roboto.html">
  <link rel="import"
    href="../components/core-header-panel/core-header-panel.html">
  <link rel="import"
    href="../components/core-toolbar/core-toolbar.html">
  <link rel="import" href="tab-list.html">
  <link rel="import" href="post-list.html">
</head>

<body unresolved touch-action="auto">
  <core-header-panel>
    <core-toolbar>
      <tab-list></tab-list>
    </core-toolbar>
    <div class="container" layout vertical center>
      <post-list show="all"></post-list>
    </div>
  </core-header-panel>

  <script>

  var list = document.querySelector('post-list');

  </script>
</body>

</html>

Mas

querySelector('paper-tabs') = *null*

Eu tentei colocar oeventListener noindex.html mas eu tenho o mesmo problema alguém pode me dizer onde está o problema?

Muito obrigado!

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