MultiConverter returns a value but the value is not set in the width property

Question

I have a piece of code sample which suppose to set the width of the rectangle from a converter. I can see the converter is called twice (the second time with the required value) but the value does not seem to reach the Width property of the rectangle.

note: I use rectangle on purpose instead of a progressbar - requirements

Here is the xaml

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:viewmodel="clr-namespace:WpfApplication1"
    Title="MainWindow" Height="350" Width="525">
<Window.Resources>
    <viewmodel:CalcWidthFromPercentageConverter x:Key="CalcWidthFromPercentageConverter" />
</Window.Resources>
<Window.DataContext>
    <viewmodel:ViewModel/>
</Window.DataContext>
<Grid>
    <Border x:Name="borderParent" Height="20" HorizontalAlignment="Stretch" BorderBrush="Black" BorderThickness="1" VerticalAlignment="Center">
        <Rectangle Fill="Red" Height="20" HorizontalAlignment="Left">
            <Rectangle.Width>
                <MultiBinding Converter="{StaticResource CalcWidthFromPercentageConverter}">
                    <Binding Path="ProgressWidthPercentage"/>
                    <Binding Path="ActualWidth" ElementName="borderParent"></Binding>
                </MultiBinding>
            </Rectangle.Width>
        </Rectangle>
    </Border>
</Grid>

here is the converter

public class CalcWidthFromPercentageConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
    {
        if (values.Count() == 2)
        {
            decimal inputPercentage = (decimal)values[0];
            decimal borderWidth = System.Convert.ToDecimal(values[1]);
            decimal result = borderWidth * inputPercentage;
            return result;
        }
        return values;
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
    {
        return new[] { value };
    }
}

and last the viewmodel

public class ViewModel
{
    private readonly decimal _progressWidthPercentage;

    public decimal ProgressWidthPercentage
    {
        get { return _progressWidthPercentage; }
    }

    public ViewModel()
    {
        _progressWidthPercentage = (decimal)0.37;
    }
}

Answer 1

Just a copy of "JanW" comment which solved this:

"Two things i would suggest you: Width is measured in Double. Use Double instead of Decimal. More important: do not return "values" as default, since it does not deliver a valid Double value, instead object[]. This would cause the binding to fail"

I just had to turn the value binding and returned in the converter to double instead of decimal.