animating combobox popup

Hi,

i am trying to animate the popup of a combobox with this controltemplate :

<Popup x:Name="PART_Popup"
                   AllowsTransparency="True"
                   Grid.ColumnSpan="2"
                   RenderTransformOrigin="0.5,0.5"
                   IsOpen="{Binding IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
                   Margin="1"
                   // removed the default animation
                   Placement="Center"
                   VerticalAlignment="Center"><Popup.RenderTransform><TransformGroup><ScaleTransform ScaleY="1"
                                        ScaleX="1"

.....

<Trigger Property="IsDropDownOpen"
                     Value="True"><Trigger.EnterActions><BeginStoryboard><Storyboard><DoubleAnimation Storyboard.TargetName="PART_Popup"
                                             Storyboard.TargetProperty="(ScaleTransform.ScaleX)"
                                             Duration="0:0:3"
                                             From="0"
                                             To="2" /><DoubleAnimation Storyboard.TargetName="PART_Popup"
                                             Storyboard.TargetProperty="(ScaleTransform.ScaleY)"
                                             Duration="0:0:3"
                                             From="0"
                                             To="2" /></Storyboard></BeginStoryboard></Trigger.EnterActions></Trigger>

but it doesn't work, the popup shows immediatly after i click on the combobox .

any idea please on how to do that please ?