WPF Popup UI showing black

I am using a WPF Popup control, and it is showing the background as black. I put a StackPanel inside it with Background="Transparent", but that does not help.

<Popup PlacementTarget="{Binding ElementName=parentStackPanel}" Placement="Center"
        IsOpen="False" Name="m_popWaitNotifier" PopupAnimation="None" 
        AllowsTransparency="False">
    <StackPanel Orientation="Vertical" Background="Transparent">
        <uc:CircularProgressBar x:Name="CB" StartupDelay="0"
                                            RotationsPerMinute="20"
                                            Height="25" Foreground="White"
                                            Margin="12"/>
    </StackPanel>
</Popup>

How does one make the background on Popup transparent (or any color)?