我试图计算移动平均线（3年），但我得到了一个不同大小的对象。

counts = dfen.groupby(['From date', 'Policy category 1']).size().reset_index(name='counts')
t = counts['counts']
counts['t_average'] = t.rolling(2).mean()

我得到：

ValueError                                Traceback (most recent call last)
~\AppData\Local\Temp\ipykernel_6032\1024416527.py in <module>
2 counts = dfen.groupby(['From date', 'Policy category 1']).size().reset_index(name='counts')
3 t = counts['counts']
4 counts[t_average] = t.rolling(2).sum()
5 
6 

~\anaconda3\lib\site-packages\pandas\core\frame.py in __setitem__(self, key, value)
3641             self._setitem_frame(key, value)
3642         elif isinstance(key, (Series, np.ndarray, list, Index)):
3643             self._setitem_array(key, value)
3644         elif isinstance(value, DataFrame):
3645             self._set_item_frame_value(key, value)

~\anaconda3\lib\site-packages\pandas\core\frame.py in _setitem_array(self, key, value)
3700 
3701             else:
3702                 self._iset_not_inplace(key, value)
3703 
3704     def _iset_not_inplace(self, key, value):

~\anaconda3\lib\site-packages\pandas\core\frame.py in _iset_not_inplace(self, key, value)
3719         if self.columns.is_unique:
3720             if np.shape(value)[-1] != len(key):
3721                 raise ValueError("Columns must be same length as key")
3722 
3723             for i, col in enumerate(key):

ValueError: Columns must be same length as key

有人知道我做错了什么吗？先谢了
我读了Pandas的文档，它说NaN是自动填充的开始和结束值（Iàm计算一个居中的平均值）...和文档中的例子是一样的...（https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.rolling.html）我也搜索了Stackoverflow，但没有看到这个问题，所以我很困惑。