Drop event not firing for stackpanel in wpf - wpf

Question

I have a grid, which contains a Canvas and StackPanel on same level. I mean the StackPanel placed on the top of the canvas. The canvas contains multiple elements which should be dragable. And I have to handle the Drop event of the StackPanel. The drop event not working if the stackpanel does not contain any child elements, even if I set the height for stackpanel. If I set the Background property for StackPanel it will work, but in this case the mousemove events of canvas child elements(below stackpanel) not working.
Here is the XAML
<Grid>
<jas:DragCanvas
x:Name="dragCanvas"
Grid.Row="1"
AllowDragging="{Binding ElementName=btnAllowDragging, Path=IsChecked}"
AllowDragOutOfView="{Binding ElementName=btnAllowDragOutOfView, Path=IsChecked}"
>
<Ellipse
Canvas.Left="100" Canvas.Bottom="100"
Width="65" Height="70">
<Ellipse.Fill>
<LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
<LinearGradientBrush.GradientStops>
<GradientStop Color="LightGray" Offset="0" />
<GradientStop Color="Black" Offset="0.75" />
</LinearGradientBrush.GradientStops>
</LinearGradientBrush>
</Ellipse.Fill>
</Ellipse>
</jas:DragCanvas>
<DockPanel LastChildFill="False">
<Border DockPanel.Dock="Left" BorderBrush="Black" BorderThickness="2">
<StackPanel x:Name="stackPanel" Drop="StackPanel_Drop" AllowDrop="True">
<Border Height="100" Width="100" Background="Red"></Border>
</StackPanel>
</Border>
</DockPanel>
</Grid>