Получение данных с разных элементов Slider в WPF

Утро доброе, появилась необходимость взять данные с 2 разных слайдеров, тип данных Double: Пробовал взять данные так:

double srok = Slider1_Copy.Value;
double summa = Slider1.Value;

Но получается исключение как на скриншоте:

Как можно вытащить данные? Весь код обработчика ниже:

private void SliderXXX(object sender, RoutedPropertyChangedEventArgs<double> e)
    {
        double val;
        val = (sender as Slider).Value;

        double srok = Slider1_Copy.Value;
        double summa = Slider1.Value;

        // Стабильный вклад 9,85% Без Кап. Без Попол. Без Снятия.

            // Расчет процента умножения вклада

             double koef = srok / 365;
             double percent = 9.85 * koef;

        // Конец расчета умножения вклада

        double percent_1 = summa / 100;
        double dohod = percent_1 * koef;
        double itog = summa + dohod;

        string dohod1 = dohod.ToString();
        string itog1 = itog.ToString();

        MessageBox.Show(dohod1);
        MessageBox.Show(itog1);


        // END Стабильный вклад

    }

XAML:

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition/>
        <ColumnDefinition/>
    </Grid.ColumnDefinitions>
    <Label Content="Рассчитать доход" HorizontalAlignment="Left" Margin="30,10,0,0" VerticalAlignment="Top" FontFamily="Times New Roman" FontSize="22"/>
    <TextBox Text="{Binding ElementName=Slider1, Path=Value, UpdateSourceTrigger=LostFocus}" HorizontalAlignment="Left" Margin="18,87,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="194" Height="25" FontSize="16"/>
    <Label Content="Сумма" HorizontalAlignment="Left" Margin="18,56,0,0" VerticalAlignment="Top" FontSize="16" FontFamily="Times New Roman"/>
    <Slider ValueChanged="SliderXXX" Name="Slider1" TickFrequency="100" IsSnapToTickEnabled="True" Minimum="1000" Maximum="10000000" HorizontalAlignment="Left" Margin="18,117,0,0" VerticalAlignment="Top" Width="194" Value="1000"/>
    <Label Content="Руб." HorizontalAlignment="Left" Margin="172,81,0,0" VerticalAlignment="Top" FontSize="16"/>
    <Label Content="Срок" HorizontalAlignment="Left" Margin="18,168,0,0" VerticalAlignment="Top" FontSize="16" FontFamily="Times New Roman"/>
    <TextBox Text="{Binding ElementName=Slider1_Copy, Path=Value, UpdateSourceTrigger=LostFocus}" HorizontalAlignment="Left" Margin="18,196,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="194" Height="25" FontSize="16"/>
    <Label Content="дней" HorizontalAlignment="Left" Margin="166,190,0,0" VerticalAlignment="Top" FontSize="16"/>
    <Slider ValueChanged="SliderXXX" x:Name="Slider1_Copy" TickFrequency="1" IsSnapToTickEnabled="True" Minimum="30" Maximum="1825" HorizontalAlignment="Left" Margin="18,229,0,0" VerticalAlignment="Top" Width="194" Value="30"/>
    <Label Content="Ежемесячное пополнение" HorizontalAlignment="Left" Margin="18,280,0,0" VerticalAlignment="Top" FontSize="16" FontFamily="Times New Roman"/>
    <TextBox Text="{Binding ElementName=Slider1_Copy1, Path=Value, UpdateSourceTrigger=LostFocus}" HorizontalAlignment="Left" Margin="18,308,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="194" Height="25" FontSize="16"/>
    <Label Content="Руб." HorizontalAlignment="Left" Margin="172,305,0,0" VerticalAlignment="Top" FontSize="16"/>
    <Slider ValueChanged="SliderXXX" x:Name="Slider1_Copy1" TickFrequency="100" IsSnapToTickEnabled="True" Minimum="0" Maximum="5000000" HorizontalAlignment="Left" Margin="18,341,0,0" VerticalAlignment="Top" Width="194" Value="1000"/>


    <!--BORDER-->

    <Border BorderBrush="Black" BorderThickness="0.3" Grid.Column="2" Margin="30,30,30,30" Background="White">
        <Border.Effect>
            <DropShadowEffect Color="Black" Direction="330" ShadowDepth="0" Opacity="0.4" BlurRadius="30"/>
        </Border.Effect>
        <TextBox Name="TBStable" TextWrapping="Wrap" Text="TextBox" Margin="10,10,224,340"/>
    </Border>

    <!--BORDER-->


</Grid>

Форма: