ResourceDictionary error in Silverlight

MainPage.xaml

<UserControl x:Class="ResourceDictionaryHeadache.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
    mc:Ignorable="d" d:DesignWidth="640" d:DesignHeight="480">
    <UserControl.Resources>
        <ResourceDictionary Source="/SampleData.xaml" />
    </UserControl.Resources>
    <Grid x:Name="LayoutRoot">
        <ListBox HorizontalAlignment="Stretch"
                 VerticalAlignment="Stretch"
                 ItemsSource="{StaticResource SampleData}">
        </ListBox>
    </Grid>
</UserControl>

Person.cs

using System;
using System.Collections.ObjectModel;

namespace ResourceDictionaryHeadache
{
    public class PersonList : ObservableCollection<Person> { }

    public class Person
    {
        public string Name { get; set; }
        public int Age { get; set; }
    }
}

SampleData.xaml

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    xmlns:Headache="clr-namespace:ResourceDictionaryHeadache">
    <Headache:PersonList x:Key="SampleData">
        <Headache:Person Name="Joe" Age="20" />
        <Headache:Person Name="Sam" Age="25" />
        <Headache:Person Name="Dave" Age="30" />
    </Headache:PersonList>
</ResourceDictionary>

Posted as question on Stackoverflow: http://stackoverflow.com/questions/2025152/error-using-resourcedictionary-in-silverlight