Key error when indexing two Panda DataFrames with same index

Question

Python 2.7, spyder, all packages installed by Anaconda. The code runs good befor upgrade to pandas 0.20.3

Trying to index two dataframes, days and udly, with unicode string date '2017-09-13'

they both have same index

    udly.index[-1]

    Out[13]: Timestamp('2017-09-13 00:00:00')

    days.index[-1]

    Out[14]: Timestamp('2017-09-13 00:00:00')

while indexing dataframe udly, everything goes well

    udly['2017-09-13']

    Out[15]: 
        CLOSE       VOLUME   ytm_1  ATMstk
    2017-09-13  2.739  307060583.0  3.4912    2.75

But when index dataframe days, things goes tricky

    days['2017-09-13']
    Traceback (most recent call last):
    ...........
      File "pandas\_libs\index.pyx", line 132, in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5280)

      File "pandas\_libs\index.pyx", line 154, in pandas._libs.index.IndexEngine.get_loc (pandas\_libs\index.c:5126)

      File "pandas\_libs\hashtable_class_helper.pxi", line 1210, in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas\_libs\hashtable.c:20523)

      File "pandas\_libs\hashtable_class_helper.pxi", line 1218, in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas\_libs\hashtable.c:20477)

    KeyError: '2017-09-13'

Update

Indexing with .loc works, but returns me a Pandas.Series, but I would really like to know the reason why this happens, why I cannot index a dataframe.

type(udly['2017-09-13'])
Out[23]: pandas.core.frame.DataFrame

type(days.loc['2017-09-13'])
Out[24]: pandas.core.series.Series

even trickier part is that I can index dataframe days with partical index such as '2017-09' and it works, but I really need to index a specific day.

days['2017-09']
Out[39]: 

2017-09-11 2017-09-11
2017-09-12 2017-09-12
2017-09-13 2017-09-13