When I convert silverlight 3.0 project to 4.0 then I am getting resource dictionary error ?

Question

When I convert silverlight 3.0 project to 4.0 then I am getting resource dictionary following error.

The file is not part of the project or its 'Build Action' property is not set to 'resource'

My app.xaml code is below for the resource dictionary.

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