def test_apply_iteration(self):
        # #2300
        N = 1000
        ind = pd.date_range(start="2000-01-01", freq="D", periods=N)
        df = DataFrame({'open': 1, 'close': 2}, index=ind)
        tg = TimeGrouper('M')

        _, grouper, _ = tg._get_grouper(df)

        # Errors
        grouped = df.groupby(grouper, group_keys=False)
        f = lambda df: df['close'] / df['open']

        # it works!
        result = grouped.apply(f)
        self.assertTrue(result.index.equals(df.index))