using radlineargauge always gets does not support direct content

I always get this error but is able to eventually build and compile:

Severity Code Description Project File Line Suppression State
Error  The type 'RadLinearGauge' does not support direct content. TestingWorkstation C:\Users\Victor\Desktop\TestingWorkstationUWP-010216\TestingWorkstation\PDL_Page.xaml 23 

This is the code causing the problem:

                    <DataVisualization:RadLinearGauge HorizontalAlignment="Left" Background="Aqua" VerticalAlignment="Top" Height="50"
                            MinValue="{Binding SliderDisplayTromboneMin,Mode=TwoWay}" MaxValue="{Binding SliderDisplayTromboneMax,Mode=TwoWay}"
                TickStep=".5" LabelStep=".5" Margin="10,0,10,0" Width="700" Foreground="DarkBlue" >

--> PROBLEM LINE -->                            <DataVisualization:LinearBarGaugeIndicator Value="{Binding TromboneMeasuredValuePS,Mode=TwoWay}" Brush="Green" Thickness="10"/>

                        </DataVisualization:RadLinearGauge>

Why do I keep getting this error message?