Python学的很浅，有点搞不懂为什么会这样。

def random_stocks_return(start_date, end_date, n):
    ret_list = pd.DataFrame()
    for c in random_pick_stocks_code(n):
        st = collect_stock_data(c,start_date,end_date)   
        st_ret = tick2ret(st['close']).to_frame(name = c)
        ret_list = pd.concat([ret_list, st_ret], axis = 1 ,join = 'outer', ignore_index = False, sort = False)
    ret_list.dropna(axis = 0, how = 'any', inplace = True)
    return ret_list
运行时候会出现TypeError: 'NoneType' object is not subscriptable