I'm trying to create a ResourceDictionary inside a WPF UserControl Library project. When I add the following style:

<Style TargetType="{x:Type Button}">
    <Setter Property="Background" Value="{StaticResource ResourceKey=GreyBrush}"/>
    <Setter Property="BorderBrush" Value="{StaticResource ResourceKey=LightBlueBrush}"/>
    <Setter Property="BorderThickness" Value="1"/>
    <Style.Triggers>
        <Trigger Property="IsMouseOver" Value="True">
            <Setter Property="Background" Value="{StaticResource ResourceKey=OrangeBrush}"/>
        </Trigger>
        <EventTrigger RoutedEvent="Click">
            <BeginStoryboard>
                <Storyboard>
                    <ColorAnimation Storyboard.TargetProperty="Background.Color" To="{StaticResource ResourceKey=LightOrange}" Duration="0:0:.1"/>
                </Storyboard>
            </BeginStoryboard>
        </EventTrigger>
    </Style.Triggers>
</Style>

I get an error saying:

The type 'x:Type' was not found. Verify that you are not missing an assembly reference and that all referenced assemblies have been built.

I am declaring x as:

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

This works when I create a resource dictionary inside a WPF application project, but not inside a UserControl Library project. Any idea why?

c# wpf user-controls resourcedictionary
Edit
Report