SettingWithCopyWarning向数据框中添加空列时出错

我有一个有两列的数据帧。我正在尝试使用以下指令添加新的空列:

df['new_col'] = ''

列已添加,但带有警告:

<ipython-input-16-779f11834aea>:1: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy

我到底做错了什么?为什么会触发此警告?

提前谢谢