Selected Value return False instead of the item

am triyng to you WPF binding it return False instead of my the item I selected

this is my Xaml: code:

ComboBox Name="cbKeyboardLayout" ItemsSource="{Binding AvailableKeyboardLayouts}" HorizontalAlignment="Right" Margin="0,2,20,2" SelectedItem="{Binding Path=SelectedKeyboardLayout, Mode=TwoWay, Converter={StaticResource kbConverter}, FallbackValue=English}" Width="120"

and this is the return

    /// <summary>
    /// Get/set the language setting for this keyboard.
    /// </summary>
    public string SelectedKeyboardLayout
    {
        get
        {
            //Debug.WriteLine("KeyboardViewModel.SelectedKeyboardLayout.get, returning " + _selectedKeyboardLayout);
            return _selectedKeyboardLayout;
        }
        set
        {
            //Debug.WriteLine("KeyboardViewModel.SelectedKeyboardLayout.set, from " + _selectedKeyboardLayout.ToString() + " to " + value.ToString());

            _selectedKeyboardLayout = value;

                if (_selectedKeyboardLayout != "Unknown")
                {
                    SetKeyAssignmentsTo(_selectedKeyboardLayout);
                    VirtualKeyboardRegistryService.TheVirtualKeyboard.TheKeyboardLayout = value;
                    Notify("SelectedKeyboardLayout");
                }

            SetFocusOnTarget();
        }
    }