GTK ComboBox set_active_text: assertion `GTK_IS_COMBO_BOX (combo_box)' failed
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I am adding a MS Word-esque Combo Box to my application to allow the user to change the font size. Here is the setup code:

    sizeBox = gtk.combo_box_new_text()
    for size in xrange(MIN_FONT_SIZE, MAX_FONT_SIZE+1):
        sizeBox.append_text(str(size))

    def UpdateFontSize(c, box):
        newSize = box.get_active_text()
        if newSize:
            self.textViewer.SetFontSize(int(newSize))

    print sizeBox.get_active_text()
    sizeBox.connect("changed", UpdateFontSize, sizeBox)

Running my program and trying to change the selected entry in the combo box gives me this:

    None
    OutNav.py:876: GtkWarning: gtk_combo_box_get_active_text: assertion `GTK_IS_COMBO_BOX (combo_box)' failed
       newSize = box.get_active_text()

So get_active_text works fine in the enclosing scope, but when I supply the ComboBox as an argument to the callback, it suddenly stops working. Can anyone tell me what is going on here? (I wanted to use a ComboBoxText as apparently gtk.combo_box_new_text is deprecated, but for some reason it isn't in my distribution of PyGTK 2.24)