In a Silverlight 4 project I have a class that extends Canvas:

public class AppendageCanvas : Canvas
{        
    public float Friction { get; set; }
    public float Restitution { get; set; }
    public float Density { get; set; }
}

I use this canvas in Blend by dragging it onto another control and setting the custom properties:

When I run the app, I get the following error when InitializeComponent is called on the control containing my custom canvas:

I'm not sure why Silverlight isn't able to convert this property from it's string representation in Xaml, to the float that it is.

Anyone have any ideas?