Set function not working.

I am having trouble with the "set" function

Error in GUIfunction>Evaluate (line 101)
    set(Bn,'String',num2str(Bn1));

Error while evaluating UIControl Callback

uicontrol('Style','pushbutton','String','Evaluate','Position',[130,550,100,50],'Callback',{@Evaluate},'BackgroundColor','green');

Bn = uicontrol('Style','edit','String','','position',[10 450 100 20]);
set(f,'Visible','on');

handles = struct('Bn',Bn);
guidata(f, handles);
function Evaluate(hObject, eventdata, handles)%#ok
    handles = guidata(hObject);
    Bn = handles.Bn;
    Bn1=40;    
    set(Bn,'String',num2str(Bn1));

[Walter Roberson]

There should be at least one more line of error message before the ones you show, indicating the kind of problem that is being encountered, like no such property or not an object