Hi, this piece of code causes crash, but I do not know, if I am doing something wrong or it is just bug

def compare(model, row1, row2, user_data):
    sort_column, dummy = model.get_sort_column_id()
    value1 = model.get_value(row1, sort_column)
    value2 = model.get_value(row2, sort_column)
    if value1 <  value2:
        return 1
    elif value1 == value2:
        return 0
    else:
        return -1

model = Gtk.ListStore(str, str)
model.set_sort_func(0, compare, None)
model.set_sort_column_id(0, Gtk.SortType.DESCENDING)
model.append(['20', 'Newton'])
model.append(['20', 'Einstein'])
model.append(['20', 'Tesla'])
model.append(['25', 'Shuttleworth'])