ListPicker selected item not changing when selected in Full Mode

public class Category : INotifyPropertyChanged, INotifyPropertyChanging
{
    private int _id;
    public int Id
    {
        get { return _id; }
        set
        {
            if (_id == value) return;
            NotifyPropertyChanging();
            _id = value;
            NotifyPropertyChanged();
        }
    }

    private string _description;
    public string Description
    {
        get { return _description; }
        set
        {
            if (_description == value) return;
            NotifyPropertyChanging();
            _description = value;
            NotifyPropertyChanged();
        }
    }

    #region INotifyPropertyChanging and INotifyPropertyChanged Members
    public event PropertyChangingEventHandler PropertyChanging;
    /// <summary>
    /// Used to notify the data context that a property is about to change...
    /// </summary>
    private void NotifyPropertyChanging([CallerMemberName] string propertyName = null)
    {
        if (PropertyChanging != null) PropertyChanging(this, new PropertyChangingEventArgs(propertyName));
    }

    public event PropertyChangedEventHandler PropertyChanged;
    /// <summary>
    /// Used to notify the data context that a property has changed...
    /// </summary>
    private void NotifyPropertyChanged([CallerMemberName] string propertyName = null)
    {
        if (PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
    #endregion
}

<phone:PhoneApplicationPage.Resources>
    <DataTemplate x:Key="FullModeItemTemplate">
            <TextBlock d:DataContext="{Binding}" 
                       Text="{Binding Description}"
                       />
    </DataTemplate>
</phone:PhoneApplicationPage.Resources>

<phone:PhoneApplicationPage
    x:Class="QuestionPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:phone="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone"
    xmlns:shell="clr-namespace:Microsoft.Phone.Shell;assembly=Microsoft.Phone"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:toolkit="clr-namespace:Microsoft.Phone.Controls;assembly=Microsoft.Phone.Controls.Toolkit"
    FontFamily="{StaticResource PhoneFontFamilyNormal}"
    FontSize="{StaticResource PhoneFontSizeMedium}"
    Foreground="{StaticResource PhoneForegroundBrush}"
    SupportedOrientations="Portrait" Orientation="Portrait"
    mc:Ignorable="d"
    shell:SystemTray.IsVisible="True">

    <!--LayoutRoot is the root grid where all page content is placed.-->
    <Grid x:Name="LayoutRoot" removed="Transparent">
        <Border Style="{StaticResource ButtonBorderStyle}"
                removed="Blue"
                BorderBrush="White"
                >
            <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition Height="Auto"/>
                    <RowDefinition Height="*"/>
                </Grid.RowDefinitions>
                <!--TitlePanel contains the name of the app and page title.-->
                <StackPanel x:Name="TitlePanel" Grid.Row="0" Margin="0,10,0,0">
                    <TextBlock x:Name="ApplicationTitle" 
                        Text="pagetitle" 
                        Style="{StaticResource PhoneTextLargeStyle}"/>
                    <TextBlock x:Name="PageTitle" 
                        Text="subtitle" 
                        Style="{StaticResource  PhoneTextExtraLargeStyle}"/>
                </StackPanel>
                <StackPanel x:Name="ContentPanel" Grid.Row="1" Margin="0,0,0,15">
                    <TextBlock Text="question" Margin="20,5,0,0"/>
                    <TextBox x:Name="ItemDescription"/>
                    <TextBlock Text="category" Margin="20,0,0,-10"/>
                    <toolkit:ListPicker x:Name="CategoriesListPicker"
                                        DataContext="{Binding}" ItemsSource="{Binding Categories}"
                                        DisplayMemberPath="Description"
                                        FullModeHeader="Categories:"
                                        FullModeItemTemplate="{StaticResource FullModeItemTemplate}"
                                        CacheMode="BitmapCache"
                                        >
                    </toolkit:ListPicker>
                </StackPanel>
            </Grid>
        </Border>
    </Grid>


</phone:PhoneApplicationPage>