AttachedPropertyに設定される値がFrameworkElementを継承していない場合にUri型に値を設定できない？

MainPage.xaml

<Page
    x:Class="App1.MainPage"
    IsTabStop="false"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:App1"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
    </Grid>

    <local:Container.c1>
        <local:c1 uri="http://www.example.com/" />
    </local:Container.c1>
    <local:Container.c2>
        <local:c2 uri="http://www.example.com/" />
    </local:Container.c2>
</Page>

MainPage.xaml.cs

using Windows.UI.Xaml.Controls;

namespace App1
{
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            this.InitializeComponent();
        }
    }

    public class c1
    {
        public Uri uri { get; set; }
    }

    public class c2 : FrameworkElement
    {
        public c2(){}
        public Uri uri { get; set; }
    }

    public class Container
    {
        public static c1 Getc1(DependencyObject obj)
        {
            return (c1)obj.GetValue(c1Property);
        }

        public static void Setc1(DependencyObject obj, c1 value)
        {
            obj.SetValue(c1Property, value);
        }

        public static c2 Getc2(DependencyObject obj)
        {
            return (c2)obj.GetValue(c2Property);
        }

        public static void Setc2(DependencyObject obj, c2 value)
        {
            obj.SetValue(c2Property, value);
        }

        public static readonly DependencyProperty c1Property =
            DependencyProperty.RegisterAttached("c1", typeof(c1), typeof(Container), new PropertyMetadata(null));
        public static readonly DependencyProperty c2Property =
            DependencyProperty.RegisterAttached("c2", typeof(c2), typeof(Container), new PropertyMetadata(null));
    }
}