这段有条件地计算Pandas数据帧列数的代码有什么问题？

import pandas as pd

def compute_columns(df):
    if len(df) > 0:
        return len(df.columns)
    else:
        return 0

import pandas as pd

def compute_columns(df):
    if df is None or not isinstance(df, pd.DataFrame):
        raise ValueError("Input is not a valid Pandas DataFrame.")
    
    if df.empty:
        return 0
    else:
        return df.shape[1]