title: TypeError: an integer is required in pd.merge python
body: <p>I'm trying to combine two panda dataframes as shown below</p>
  
  <p>df_aviris</p>
  
  <pre><code>            0    1    2         3          4
  0         0.0  0.0  0.0  482636.5  4155009.5
  1         0.0  0.0  0.0  482637.5  4155009.5
  2         0.0  0.0  0.0  482638.5  4155009.5
  3         0.0  0.0  0.0  482639.5  4155009.5
  4         0.0  0.0  0.0  482640.5  4155009.5
  5         0.0  0.0  0.0  482641.5  4155009.5
  6         0.0  0.0  0.0  482642.5  4155009.5
  7         0.0  0.0  0.0  482643.5  4155009.5
  8         0.0  0.0  0.0  482644.5  4155009.5
         ...  ...  ...       ...        ...
  
  16730996  0.0  0.0  0.0  485932.5  4149940.5
  16730997  0.0  0.0  0.0  485933.5  4149940.5
  16730998  0.0  0.0  0.0  485934.5  4149940.5
  16730999  0.0  0.0  0.0  485935.5  4149940.5
  [16731000 rows x 5 columns]
  </code></pre>
  
  <p>df_geomap</p>
  
  <pre><code>              0      1      2         x          y
  0         255.0  255.0  255.0  477642.5  4158373.5
  1         255.0  255.0  255.0  477643.5  4158373.5
  2         255.0  255.0  255.0  477644.5  4158373.5
  3         255.0  255.0  255.0  477645.5  4158373.5
  4         255.0  255.0  255.0  477646.5  4158373.5
  5         255.0  255.0  255.0  477647.5  4158373.5
  6         255.0  255.0  255.0  477648.5  4158373.5
           ...    ...    ...       ...        ...
  
  79026747  255.0  255.0  255.0  487218.5  4150124.5
  79026748  255.0  255.0  255.0  487219.5  4150124.5
  79026749  255.0  255.0  255.0  487220.5  4150124.5
  [79026750 rows x 5 columns]
  </code></pre>
  
  <p>I tried to merge these two based on x and y. </p>
  
  <pre><code>DFinal = pd.merge(df_aviris,df_geomap,how='outer',on=['x','y'],left_index=False,right_index=False,copy=False)
  </code></pre>
  
  <p>and using concat also</p>
  
  <pre><code>DFinal = pd.concat([df_aviris.set_index(['x','y']),df_geomap.set_index(['x','y'])],join='inner',axis=1)
  </code></pre>
  
  <p>But getting the error as below</p>
  
  <pre><code>Traceback (most recent call last):
    File "pandas/index.pyx", line 154, in pandas.index.IndexEngine.get_loc (pandas/index.c:4279)
    File "pandas/src/hashtable_class_helper.pxi", line 404, in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8543)
  TypeError: an integer is required
  During handling of the above exception, another exception occurred:
  Traceback (most recent call last):
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/indexes/base.py", line 2134, in get_loc
      return self._engine.get_loc(key)
    File "pandas/index.pyx", line 132, in pandas.index.IndexEngine.get_loc (pandas/index.c:4433)
    File "pandas/index.pyx", line 156, in pandas.index.IndexEngine.get_loc (pandas/index.c:4363)
  KeyError: 'x'
  During handling of the above exception, another exception occurred:
  Traceback (most recent call last):
    File "pandas/index.pyx", line 154, in pandas.index.IndexEngine.get_loc (pandas/index.c:4279)
    File "pandas/src/hashtable_class_helper.pxi", line 404, in pandas.hashtable.Int64HashTable.get_item (pandas/hashtable.c:8543)
  TypeError: an integer is required
  During handling of the above exception, another exception occurred:
  Traceback (most recent call last):
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 2881, in run_code
      exec(code_obj, self.user_global_ns, self.user_ns)
    File "&lt;ipython-input-38-0a4bfba1b1f4&gt;", line 1, in &lt;module&gt;
      DFinal = pd.concat([df_aviris.set_index(['x','y']),df_geomap.set_index(['x','y'])],join='inner',axis=1)
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/core/frame.py", line 2917, in set_index
      level = frame[col]._values
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/core/frame.py", line 2059, in __getitem__
      return self._getitem_column(key)
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/core/frame.py", line 2066, in _getitem_column
      return self._get_item_cache(key)
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/core/generic.py", line 1386, in _get_item_cache
      values = self._data.get(item)
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/core/internals.py", line 3543, in get
      loc = self.items.get_loc(item)
    File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pandas/indexes/base.py", line 2136, in get_loc
      return self._engine.get_loc(self._maybe_cast_indexer(key))
    File "pandas/index.pyx", line 132, in pandas.index.IndexEngine.get_loc (pandas/index.c:4433)
    File "pandas/index.pyx", line 156, in pandas.index.IndexEngine.get_loc (pandas/index.c:4363)
  KeyError: 'x'
  </code></pre>
  
  <p>I'm using python 3.6.1</p>