DragEnter and Drop event won't be fired on a ToolWindow

<UserControl x:Class="Realtech.ControlCenter.Client.Modules.Analyzer.AnalyzerView"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"      
            xmlns:shared="http://schemas.actiprosoftware.com/winfx/xaml/shared"
     xmlns:docking="http://schemas.actiprosoftware.com/winfx/xaml/docking"              
             mc:Ignorable="d" 
             HorizontalAlignment="Stretch" VerticalAlignment="Stretch"
             d:DesignHeight="600" d:DesignWidth="850"
             MinHeight="200" MinWidth="200" >
   <docking:DockSite x:Name="dockSite" >
         <!-- Workspace -->
         <docking:Workspace Background="Transparent">
            <docking:TabbedMdiHost x:Name="tabbedMdiHost" >
               <docking:TabbedMdiContainer Name="tabbedMdiContainer" />
            </docking:TabbedMdiHost>
         </docking:Workspace>
   </docking:DockSite>
</UserControl>

   public partial class AnalyzerView : UserControl
   {  
      public AnalyzerView()
      {
         InitializeComponent();

         AnalyzerDocument document1 = new AnalyzerDocument(dockSite);
         AnalyzerDocument document2 = new AnalyzerDocument(dockSite);
         AnalyzerDocument document3 = new AnalyzerDocument(dockSite);
         AnalyzerDocument document4 = new AnalyzerDocument(dockSite);

         tabbedMdiContainer.Items.Add(document1);
         tabbedMdiContainer.Items.Add(document2);
         tabbedMdiContainer.Items.Add(document3);
         tabbedMdiContainer.Items.Add(document4);
      }
   }

   public class AnalyzerDocument : DocumentWindow
   {
      DockSite _workspace = null;

      public AnalyzerDocument(DockSite dockSite)
         : base(dockSite)
      {
         _workspace = new DockSite();
         Content = _workspace;

         AllowDrop = true;

         AddCustomLayoutPanel();
         AddCustomLayoutPanel();
         AddCustomLayoutPanel();
      }

      private void AddCustomLayoutPanel()
      {
         CustomLayoutPanel panel = new CustomLayoutPanel(_workspace);
      }
   }

   public class CustomLayoutPanel : ToolWindow
   {
      Canvas _workspace = null;
      Point _dropPosition = new Point();

      public CustomLayoutPanel(DockSite dockSite)
         : base(dockSite)
      {
         _workspace = new Canvas();
         Content = _workspace;

         AllowDrop = true;

         Dock();

         this.DragEnter += new DragEventHandler(OnDragOver);
         this.Drop += new DragEventHandler(OnDrop);
      }

      private void OnDragOver(object sender, DragEventArgs e)
      {
         if (e.Data.GetDataPresent(typeof(string)))
         {
            e.Effects = DragDropEffects.Copy;
         }
         else
         {
            e.Effects = DragDropEffects.None;
         }
      }

      private void OnDrop(object sender, DragEventArgs e)
      {
         _dropPosition = e.GetPosition(_workspace);

     ... do somthing...
      }
   }