Reference to non-existent field error when opening another gui from main gui

I have two guis. Gui1 has a button btn_open_gui2 clicking on which opens my gui2. Now, in gui2, I have declared a variable in the opening function as

handles.flag=1 handles.output = hObject; guidata(hObject, handles);

and I have used this flag variable in some button_callbacks, say, gui2_button1 in this same gui2. When I run gui2, everything is working fine. But when I run gui1 and open gui2 from my btn_open_gui2 and then click on gui2_button1, I get this error

Reference to non-existent field 'flag'.

[Nishat Anjum Shaikh]

I think there is some error in using handles.

Please help!!