How to properly compute URI from ValueConverter in WPF

Question

I have a simple application where I want to swap out image based on a value converter. Upon testing the images shows fine when directly bound to the code behind of a user control.

public partial class HypercombControls : Grid
{


    public static readonly DependencyProperty MoveImageSourceProperty =
        DependencyProperty.Register("MoveImageSource", typeof(Uri), typeof(HypercombControls),
            new PropertyMetadata(new Uri("pack://application:,,/Media/control-bar/move.png")));
    ...
}

<Button Click="MoveButton_OnClick" Style="{StaticResource DragModeButtonStyle}">
    <Image Source="{Binding ElementName=HyperControls, Path=MoveImageSource}">

    </Image>
</Button>

The image shows as expected. I then added a MultiBinding with a MultiValueConverter to change the image source dynamically based on some logic. The image always comes up blank when I return the URI even if I return the exact same image URI as constructed initially. Is there something I am missing?

    public object Convert(object[] values, Type targetType, object parameter, 
              CultureInfo culture)
    {
        // logic omitted but tested with the following ouput.
        // same as new PropertyMetadata(...)
        return new Uri("pack://application:,,/Media/control-bar/move.png");
    }