WPF ComboBox Undo Exception on Ctrl+Z, Why?

Well, someone can tell me why this exception occurs in these conditions?

Test Window:

<Window x:Class="MainWindow"
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 Title="MainWindow" Height="350" Width="525"><StackPanel><ComboBox
        Name="cmbTest"
        IsEditable="True"
        SelectionChanged="ComboBox_SelectionChanged"></ComboBox></StackPanel></Window>

Code behind:

Class MainWindow

  Sub New()
    ' This call is required by the designer.
    InitializeComponent()

    ' Add any initialization after the InitializeComponent() call.
    Dim source As New ObservableCollection(Of String)()
    source.Add("AAA")
    source.Add("BBB")
    source.Add("CCC")
    cmbTest.ItemsSource = source
  End Sub

  Private Sub ComboBox_SelectionChanged(sender As System.Object, e As System.Windows.Controls.SelectionChangedEventArgs)
    Keyboard.Focus(cmbTest)
  End Sub

End Class

Now, if you type "AAA" and then "Ctrl+Z", combobox control raise this exception "Cannot Undo or Redo while undo unit is open."

Someone can explain why?!? ...how it would be possible to avoid it?