Panel.apply (2d) doesn't return a result of Series in 0.16.1
Panel.apply (2d) doesn't return a result of Series in 0.16.1

y = panel.apply(lambda df: df + 1, axis=[1, 2])
--> Returns Panel. OK.

y = panel.apply(lambda df: df['Column'] + 1, axis=[1, 2])
--> Returns DataFrame. OK.

y = panel.apply(lambda df: df['Column'].mean(), axis=[1, 2])
--> Returns Series is expected. but got error as follows.

>   File "test.py", line 28, in <module>
>    y = panel.apply(lambda df: df['Column'].mean(), axis=[1, 2])
>   File "C:\Anaconda3\lib\site-packages\pandas\core\panel.py", line 969, in apply
>     return self._apply_2d(f, axis=axis)
>   File "C:\Anaconda3\lib\site-packages\pandas\core\panel.py", line 1068, in _apply_2d
>     return self._construct_return_type(dict(results))
>   File "C:\Anaconda3\lib\site-packages\pandas\core\panel.py", line 1124, in _construct_return_type
>     '[result->%s]' % (self, result))
> pandas.core.common.PandasError: invalid _construct_return_type [self-><class 'pandas.core.panel.Panel'>
