NonselectableTags when used tab in listbox don't work

<zk>
    <zscript>
        Object[] o = new Object[20];
    </zscript>
    <listbox nonselectableTags="">
        <listhead>
            <listheader label="name"></listheader>
        </listhead>
        <listitem forEach="${o}">
            <listcell>
                <textbox value="item ${forEachStatus.index}" inplace="true"></textbox>
            </listcell>
        </listitem>
    </listbox>
</zk>

<zk>
    <zscript>
        Object[] o = new Object[20];
    </zscript>
    <listbox id="listbox">
        <listhead>
            <listheader label="name"></listheader>
        </listhead>
        <listitem forEach="${o}">
            <listcell>
                <textbox tabindex="${forEachStatus.index + 1}"
                    value="item ${forEachStatus.index}" inplace="true"
                    onFocus="listbox.setSelectedIndex(self.tabindex - 1)">
                </textbox>
            </listcell>
        </listitem>
    </listbox>
</zk>