Validation with IDataErrorInfo through DependencyProperty - wpf

Question

I've created a small user control to extend my fun with validation!
The problem is that the validation does not work: IDataErrorInfo.this[] is never called.
Thank you in advance for your help.
Stephan
Here is my code:
<UserControl x:Class="WpfApplication5.TextBoxWithValidation"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
x:Name="TextBoxWithValidationControl">
<Grid>
<StackPanel Orientation="Horizontal">
<TextBox Height="23" Width="120"
Text="{Binding Path=Text,ElementName=TextBoxWithValidationControl, ValidatesOnDataErrors=True, UpdateSourceTrigger=PropertyChanged}">
</TextBox>
</StackPanel>
</Grid>
And the code behind is:
public partial class TextBoxWithValidation
{
public static readonly DependencyProperty TextProperty = DependencyProperty.Register("Text",
typeof (object),
typeof (
TextBoxWithValidation
),
new FrameworkPropertyMetadata
(default(object),
FrameworkPropertyMetadataOptions
.BindsTwoWayByDefault));
public TextBoxWithValidation()
{
InitializeComponent();
}
public object Text
{
get { return GetValue(TextProperty); }
set { SetValue(TextProperty, value); }
}
}
Now I'm using this user control as such:
<local:TextBoxWithValidation Text="{Binding Path=Dummy.MyProperty}" />
And my Dummy entity is defined by:
public class Dummy : INotifyPropertyChanged, IDataErrorInfo
{
private string _myProperty;
public string MyProperty
{
get { return _myProperty; }
set
{
_myProperty = value;
OnPropertyChanged("MyProperty");
}
}
#region IDataErrorInfo Members
public string this[string columnName]
{
get { return "This is an error"; }
}
public string Error { get; private set; }
#endregion
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
#endregion
protected virtual void OnPropertyChanged(string propertyName)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs(propertyName));
}
}