Error in acces to the handles of NewValue through the eventdata of a buttongroup

Hello,

I am trying to access to the handles of the new selection inside a buttongroup using the recomendation made in the web reference: http://www.mathworks.de/help/techdoc/creating_guis/f10-998412.html#brrll58-1

function uibuttongroup1_SelectionChangeFcn(hObject,eventdata)

switch get(eventdata.NewValue,'Tag') % Get Tag of selected object.

    case 'radiobutton1'
        % Code for when radiobutton1 is selected.
    case 'radiobutton2'
        % Code for when radiobutton2 is selected.
    case 'togglebutton1'
        % Code for when togglebutton1 is selected.
    case 'togglebutton2'
        % Code for when togglebutton2 is selected.
    % Continue with more cases as necessary.
    otherwise
        % Code for when there is no match.
end

My concrete code is:

function uipanel17_SelectionChangeFcn(hObject, eventdata, handles)

Tag_Fehler=get(eventdata.NewValue,'Tag'); % Get Tag of selected object.

switch Tag_Fehler

    case 'togglebutton2_3F'
        set(handles.text2,'String','S-C current, Ippk3  [A]')
    case 'togglebutton3_2F'
        set(handles.text2,'String','S-C current, Ippk2  [A]')
end

The problem is that I am getting this error that I don´t really uderstand why:

??? Error while evaluating uicontrol Callback

??? Attempt to reference field of non-structure array.

Error in ==> input_biegesteifen>uipanel17_SelectionChangeFcn at 1149 Tag_Fehler=get(eventdata.NewValue,'String');

Error in ==> gui_mainfcn at 96 feval(varargin{:});

Error in ==> input_biegesteifen at 42 gui_mainfcn(gui_State, varargin{:});

Error in ==> hgfeval at 65 evalin('base', fcn);

Error in ==> uitools.uibuttongroup.childAddedCbk>manageButtons at 80 hgfeval(cbk, source, evdata);

??? Error while evaluating uicontrol Callback

Does someone understands what is the problem?

Thanks a lot,

David Dapuero