Unable to cast object in WPF MultiValueConverter

Question

I am trying to learn MultiBinding. I try to pass two values (one double and one boolean) from viewmodel to the converter and return a string result back.

XAML

<TextBlock>
     <TextBlock.Text>
         <MultiBinding Converter="{StaticResource MyValueConverter}">
              <Binding RelativeSource="{RelativeSource Mode=FindAncestor, AncestorType=ScrollViewer}" Path="DataContext.SelectedCar.PowerValue" />
              <Binding RelativeSource="{RelativeSource Mode=FindAncestor, AncestorType=ScrollViewer}" Path="DataContext.SelectedCar.IsPowerAvailable" />
         </MultiBinding>
     </TextBlock.Text>
</TextBlock>

Converter

class MyValueConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        if (System.Convert.ToBoolean(values[1]))
        {
            return string.Format("{0:n1}", values[0]); 
        }
        else
        {
            return "[not available]";
        }
    }
    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

Viewmodel

public double PowerValue
public bool IsPowerAvailable

In the converter I will get an exception Unable to cast object of type 'MS.Internal.NamedObject' to type 'System.IConvertible.

What does this mean? Did I implement the MultiBinding incorrectly?

EDIT:

ViewModel looks like this

class CarViewModel : ViewModelBase
{
    private car _selectedCar;
    public car SelectedCar
    {
        get { return _selectedCar; }
        set
        {
            _selectedCar = value;
        }
    }

Model looks like this:

class car 
{
    public double PowerValue { get; set; }
    public bool IsPowerAvailable{ get; set; }
}

Answer 1

The exception is due to the fact that the DataContext.SelectedCar.IsPowerAvailable can't be reached, make sure that the SelectedCar property is indeed reachable from the ScrollViewer DataContext, here a working sample

 Title="MainWindow" Height="350" Width="525" 
    DataContext="{Binding RelativeSource={RelativeSource Self}}">
<Window.Resources>
    <wpfApplication12:MyValueConverter x:Key="MyValueConverter"/>
</Window.Resources>
<Grid>      
  <ScrollViewer>
        <TextBlock>
            <TextBlock.Text>
                <MultiBinding Converter="{StaticResource MyValueConverter}">
                    <Binding RelativeSource="{RelativeSource Mode=FindAncestor, AncestorType=ScrollViewer}" Path="DataContext.SelectedCar.PowerValue" />
                    <Binding RelativeSource="{RelativeSource Mode=FindAncestor, AncestorType=ScrollViewer}" Path="DataContext.SelectedCar.IsPowerAvailable" />
                </MultiBinding>
            </TextBlock.Text>
        </TextBlock>
    </ScrollViewer>
</Grid>

and the selectedCar is defined in the VM/CodeBhind:

private Car _selectedCar = new Car()
    {
        IsPowerAvailable = true,
        PowerValue = 14
    };
    public Car SelectedCar
    {
        get
        {
            return _selectedCar;
        }

        set
        {
            if (_selectedCar == value)
            {
                return;
            }

            _selectedCar = value;                
        }
    }