无法将pandas df列标题更改为小写

这是我的代码的相关部分:

df_pandas.columns = df_pandas.columns.str.lower()
df_pandas = df_pandas.rename(columns={'Date': 'date'})
df_pandas = df_pandas.reset_index( )

All the column headers changed to lowercase except Date: https://i.imgur.com/pQUmmmq.png I've tried renaming the column but that didn't work either.