How to add a generic type to a page class?

[LZD121]

I tried to add a generic type like this:

public class PageEx<T> : Page{
    {
        [Reactive]
        public T ViewModel { get; set; }

        public PageEx()
        {
            this.ViewModel = Activator.CreateInstance<T>();
        }
    }
}

namespace myApp.Views
{
    /// <summary>
    /// An empty page that can be used on its own or navigated to within a Frame.
    /// </summary>
    public sealed partial class BlankPage1 : PageEx<BlankPage1ViewModel>
    {
        public BlankPage1()
        {
            this.InitializeComponent();
        }
    }
}

<?xml version="1.0" encoding="utf-8" ?>
<infrastructure:PageEx
    x:Class="myApp.Views.BlankPage1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:infrastructure="using:myApp.Infrastructure"
    xmlns:local="using:myApp.Views"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    x:TypeArguments="local:BlankPage1ViewModel"
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
    mc:Ignorable="d">

    <Grid />
</infrastructure:PageEx>

But when I compile the app, it keeps alerting me in the BlankPage.g.i.cs :

The generic type I declared in xaml does not work..