Error in C# WPF: 'System.Windows.Style' is not a valid value for property 'System.Windows.Controls.Control.Template'

I'm using WPF MVVM pattern, and made Control's Style in MainViewResource.xaml.

Normally there is no problem, but when Slider is included in Control, the following error occurs.

'System.Windows.Style' is not a valid value for property 'System.Windows.Controls.Control.Template'.

There is no problem running the app, but Control is not displayed on the xaml screen.

MainViewResource.xaml

<Style TargetType="{x:Type Slider}" x:Key="SLD.INPUT">
    ...
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type Slider}">
                <ControlTemplate.Resources>
                    ...
                </ControlTemplate.Resources>
                <Border x:Name="border" Style="{StaticResource IN.BORDER.01}">
                    <Grid Style="{StaticResource IN.GRID}">
                        <DockPanel Style="{StaticResource IN.DOCK}">
                            <Border x:Name="PART_SelectionRange" Style="{StaticResource IN.BORDER.02}">
                                <Rectangle x:Name="sliderBar" Style="{StaticResource IN.RTG.01}"/>
                            </Border>
                            <Border Style="{StaticResource IN.BORDER.02}">
                                <Rectangle x:Name="PART_NegativeSelection" Style="{StaticResource IN.RTG.02}"/>
                            </Border>
                        </DockPanel>
                        <Track x:Name="PART_Track">
                            <Track.Thumb>
                                <Thumb x:Name="thumb" Style="{StaticResource IN.THUMB}"/>
                            </Track.Thumb>
                        </Track>
                    </Grid>
                </Border>
                <ControlTemplate.Triggers>
                    <DataTrigger Binding="{Binding ElementName=thumb, Path=IsMouseOver}" Value="true">
                        <Setter TargetName="sliderBar" Property="Fill" Value="{StaticResource LGB.SLD.FG.OVER}"/>
                    </DataTrigger>
                    <DataTrigger Binding="{Binding ElementName=thumb, Path=IsDragging}" Value="true">
                        <Setter TargetName="sliderBar" Property="Fill" Value="{StaticResource LGB.SLD.FG.DRAG}"/>
                    </DataTrigger>
                    <Trigger Property="IsEnabled" Value="false">
                        <Setter TargetName="PART_SelectionRange" Property="Visibility" Value="Collapsed"/>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<Style TargetType="{x:Type Control}" x:Key="CTL.INPUT">
    <Setter Property="Grid.Row" Value="1"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type Control}">
                <ControlTemplate.Resources>
                    <Style TargetType="{x:Type TextBlock}" x:Key="IN.CONTENT">
                        ...
                        <Setter Property="Text" Value="{Binding ElementName=slider2, Path=Value, Converter={StaticResource StringFormatConverter}}"/>
                    </Style>
                </ControlTemplate.Resources>
                <Grid>
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition Width="Auto"/>
                        <ColumnDefinition Width="*"/>
                    </Grid.ColumnDefinitions>
                    <Slider x:Name="slider2" Style="{StaticResource SLD.INPUT}"/>  // I guess here is the problem.
                    <TextBlock Style="{StaticResource IN.CONTENT}"/>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

MainWindow.xaml

<Control Style="{StaticResource CTL.INPUT}"/>

The xaml screen shows nothing as below...

However, when executed, controls are displayed properly.

MainViewResource.xaml is properly registered with the App.xaml, and the style of other controls is fine. Problems arise solely when using Slider's custom style as a StaticResource. Does anyone know what I'm missing?