Tabs content not visible

Question

I'm testing some very simple code to see how Titanium works, but I'm stuck in an apparently very simple step. Testing this sample code for the TabGroup UI element I cannot see any content (the labels) inside the tabs. If I dynamically add labels inside a tab they will work, but not if the UI elements are declared in the xml file of the view.

Reference: https://wiki.appcelerator.org/display/guides2/TabGroup

Code: app/views/index.xml

<Alloy>
    <TabGroup>
        <!-- Explicit Window declaration -->
        <Tab id="tab1">
            <Window id="win1">
                <Label>Label 1</Label>
                <Label>Label 2</Label>
                <Label platform="ios">Label 3</Label>
                <Label>Label 4</Label>
                <Label>Label 5</Label>
            </Window>
        </Tab>
        <!-- Tab included via <Require> tag -->
        <Require src="tabView"/>
    </TabGroup>
</Alloy>

app/views/tabView.xml

<Alloy>
  <Tab title="required">
    <Window title="required">
      <Label>This is a required tab</Label>
    </Window>
  </Tab>
</Alloy>

Does anyone have a hint to clear this issue?