WPF Canvas.GetLeft() always receive NaN value

I'm trying to getLeft of a control in WPF. But I always received the NaN value instead of double value. Here is what I'm trying:

XAML:

<Canvas Canvas.Left="230" Name="cnvSaver">
   <Border Focusable="False"  BorderBrush="#7FBFA379" BorderThickness="0 1" Width="460" Height="460">
       <Canvas Background="#3FFFFFFF" AllowDrop="True" Name="cnvViewerLower" Drop="cnvViewerLower_Drop" Height="460" Width="460">

       </Canvas>
   </Border>
   <Border Focusable="False" Name="borUpper" BorderBrush="#7FBFA379" BorderThickness="1 0">
       <Border.RenderTransform>
            <TranslateTransform x:Name="borUpperTranslate" X="0"/>
       </Border.RenderTransform>
       <Border.Triggers>
            <EventTrigger RoutedEvent="RadioButton.Checked">
                  <BeginStoryboard>
                       <Storyboard Name="stbUpperTranslate" Storyboard.TargetName="borUpperTranslate" Storyboard.TargetProperty="X">
                            <DoubleAnimation RepeatBehavior="Forever" Name="dbaUpperTranslate" From="230" To="0" Duration="0:0:3" AutoReverse="True"/>
                       </Storyboard>
                  </BeginStoryboard>
             </EventTrigger>
         </Border.Triggers>
         <Canvas Background="#3FEEDBB3" Drop="cnvViewerUpper_Drop" MouseLeftButtonDown="cnvViewerUpper_MouseLeftButtonDown" MouseMove="cnvViewerUpper_MouseMove" MouseLeftButtonUp="cnvViewerUpper_MouseLeftButtonUp" AllowDrop="True" Width="460" Height="460" Name="cnvViewerUpper"></Canvas>
      </Border>
</Canvas>

Code behind C#:

Point p = e.GetPosition(cnvSaver);
            if (x > p.X)
            {
                // Move to Left
                Border br = cnvViewerUpper.Parent as Border;
                double left = Canvas.GetLeft(br) - Math.Abs(p.X - x);
                if(left > 0){
                    Canvas.SetLeft(br, left);
                }
                MessageBox.Show("move to left");
            }
            else if (x == p.X)
            {
                MessageBox.Show("standing");
            }
            else {
                // Move to Right
                double left = Canvas.GetLeft(borUpper);
                if (left > 0)
                {
                    Canvas.SetLeft(borUpper, left);
                }
                MessageBox.Show("move to right");
            }

Thanks in advance.