Silverlight - XamlParseException The type 'Type' was not found

The type 'Type' was not found. [Line: 7 Position: 21]

I'm trying to dynamically generate a datatemplate. it works fine, but if I include this attribute, I get the above exception.

Width="{Binding Path=ActualWidth,RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type telerik:GridViewCell}}}"

And the complete method:

  public DataTemplate GetTextColumnTemplate(int index)
        {

            string templateValue = @"
            <DataTemplate 
            xmlns:sys=""clr-namespace:System;assembly=mscorlib""  
            xmlns:telerik=""http://schemas.telerik.com/2008/xaml/presentation"" 
            xmlns=""http://schemas.microsoft.com/client/2007""
            xmlns:x=""http://schemas.microsoft.com/winfx/2006/xaml"">
                <StackPanel>
                    <TextBox Width=""{Binding Path=ActualWidth,RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type telerik:GridViewCell}}}"" Text=""{Binding Path=V" + (index + 1).ToString() + @",Mode=TwoWay}"" AcceptsTab=""True"" AcceptsReturn=""True""/>
                </StackPanel>
            </DataTemplate>";


            return (DataTemplate)XamlReader.Load(templateValue);

        }

You have a Silverlight project. Silverlight does not support the markup extension x:Type. Ancestor bindings in Silverlight look like this:

{Binding Path=Foo, RelativeSource={RelativeSource AncestorType=UserControl}}

[Edit] And btw you can't bind to ActualWidth. You have to observe the SizeChanged event and have some handling code. You will find quite elegant solutions to this problem here: binding-to-actualwidth.