Silverlight style works at runtime but not design-time

I have a Silverlight project and a few custom styles, but the problem is these work at runtime, but not in design-time. I have merged开发者_高级运维 my styles into the App.xaml, but they seem to be ignored and I get a "The resource ... could not be resolved." message.

Any ideas where I should start looking? The properties for the styles all have the same Build Action, Custom Tool, etc, but they just fail at design-time.

EDIT: This is the contents of the App.xaml file:

<Application.Resources>
  <ResourceDictionary>
    <ResourceDictionary.MergedDictionaries>
      <ResourceDictionary Source="Assets/Styles.xaml"/>
      <ResourceDictionary Source="Assets/CoreStyles.xaml"/>
      <ResourceDictionary Source="Assets/SDKStyles.xaml"/>
      <ResourceDictionary Source="Assets/AppStyles.xaml"/>
     </ResourceDictionary.MergedDictionaries>
   </ResourceDictionary>
</Application.Resources>

The AppStyles.xaml is my custom resource dictionary.