TabbedPage doesn't show the tabs

I've tried this static tabs example: source

  <TabbedPage.Children>    
<ContentPage Title="Tab 1">
      <StackLayout Orientation="Vertical" Padding="10,0">
        <Label Text="This is the content for the first tab" />
        <Button Text="Click here" />
      </StackLayout>
    </ContentPage>
    <ContentPage Title="Tab 2">
      <Label Text="This is the content for the second tab" />
    </ContentPage>
  </TabbedPage.Children>

I've also tried by creating separate pages for each tab; but I have the same problem: the panel with the tabs is not displayed; the first page is shown in both cases but without a tabs panel, I cannot navigate to the another page.