Frozen start doesn't work

<zk>
  <label multiline="true">
    1. you should see 4th treecol, which label is "Jan 2014".
  </label>
  <tree width="300px">
    <frozen columns="1" start="3" />
    <treecols>
      <treecol label="Oct 2013" width="100px" />
      <treecol label="Nov 2013" width="100px" />
      <treecol label="Dec 2013" width="100px" />
      <treecol label="Jan 2014" width="100px" />
      <treecol label="Feb 2014" width="100px" />
    </treecols>
    <treechildren>
      <treeitem>
        <treerow>
          <treecell label="80" />
          <treecell label="90" />
          <treecell label="100" />
          <treecell label="270" />
          <treecell label="120" />
        </treerow>
      </treeitem>
    </treechildren>
  </tree>
</zk>