无法联接对象列上的Pandas数据帧

Question

第一个数据帧（名为df1）是以下数据帧（head）：

                                         id  vn                  t        pt
0  00000408662094209082085855047091571768_1   1   8/2/2016 9:52:32  category   
1  00000408662094209082085855047091571768_2   2  8/2/2016 10:28:31  category   
2  00000408662094209082085855047091571768_2   2  8/2/2016 10:29:11  category   
3  00000408662094209082085855047091571768_2   2  8/2/2016 10:29:17      home   
4  00000408662094209082085855047091571768_2   2  8/2/2016 10:29:31      home 

第二个（命名为df2）是：

^{pr2}$

当我这样做的时候：

df1.join(df2, on='id', how='left')

我得到以下错误：

C:\Users\asterios.BSQ_AD\AppData\Local\Continuum\Anaconda\lib\site-packages\pandas\core\format.py:2087: RuntimeWarning: invalid value encountered in greater
has_large_values = (abs_vals > 1e8).any()
C:\Users\asterios.BSQ_AD\AppData\Local\Continuum\Anaconda\lib\site-packages\pandas\core\format.py:2088: RuntimeWarning: invalid value encountered in less
has_small_values = ((abs_vals < 10 ** (-self.digits)) &
C:\Users\asterios.BSQ_AD\AppData\Local\Continuum\Anaconda\lib\site-packages\pandas\core\format.py:2089: RuntimeWarning: invalid value encountered in greater
(abs_vals > 0)).any()

返回的dataframe在新的'c'列上有值'NaN'，尽管我知道有匹配的id。在

为了完整起见，以下是每个数据帧的信息：

df1：

<class 'pandas.core.frame.DataFrame'>
Int64Index: 50000 entries, 0 to 49999
Data columns (total 4 columns):
id       50000 non-null object
vn       50000 non-null int64
t        50000 non-null object
pt       40411 non-null object
dtypes: int64(1), object(3)
memory usage: 2.3+ MB

df2：

<class 'pandas.core.frame.DataFrame'>
Int64Index: 29300 entries, 14 to 2294263
Data columns (total 2 columns):
id       29300 non-null object
c        29300 non-null int64
dtypes: int64(1), object(1)
memory usage: 686.7+ KB

有什么想法吗？在