WebView not working on Windows Phone

I can't make a normal WebView work on WP (it works well on Android and iOs).

I use Xamarin.Forms version 1.3.5.6337 and I am building on a Lumia 635 with WP 8.1.

This is my page:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             x:Class="MyApp.ProvaPage">
  <WebView HeightRequest="200" x:Name="webView" Source="http://www.xamarin.com/"/>
</ContentPage>

and cs file:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

using Xamarin.Forms;

namespace MyApp
{
    public partial class ProvaPage : ContentPage
    {
        public ProvaPage()
        {
            InitializeComponent();
        }
    }
}

And all I see is a blank black page...

Any idea?