Error when binding a Dependency Property on a custom Behavior

I am exploring the Silverlight attached behaviors mechanism in order to use the Model-View-ViewModel pattern within my Silverlight applications. To start with, I am trying to get a simple Hello World working, but I am completely stuck in an error for which I'm not able to find a solution.

What I have right now is a page that just contains a button which should display a message when clicked. The click event is handled by using a class derived from Behavior, and the message is specified as a dependency property of the behavior itself. The problem comes when trying to bind the message property to a property on a viewmodel class used as the data context: I get an exeption in the call to InitializeComponent in the view.

Here is all the code I'm using, as you can see it is rather simple. First the markup of the main page and the view it contains:

MyPage

<UserControl x:Class="MyExample.MyPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:MyExample"
    >
    <Grid x:Name="LayoutRoot">
        <local:MyView/>
    </Grid>
</UserControl>

MyView (the TextBlock is there just to check that the binding syntax is correct)

<UserControl x:Class="MyExample.MyView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
    xmlns:local="clr-namespace:MyExample"
    Width="400" Height="300">
    <StackPanel Orientation="Vertical" x:Name="LayoutRoot" Background="White">
        <StackPanel.Resources>
            <local:MyViewmodel x:Key="MyResource"/>
        </StackPanel.Resources>
        <TextBlock Text="Thi