---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-15-92126870e8cf> in <module>()
----> 1 data_matrix = dataset.pivot(index = 'user id').fillna(0)
2 data_matrix.head()
/Users/robertodias/Anaconda/anaconda2/lib/python2.7/site-packages/pandas/core/frame.pyc in pivot(self, index, columns, values)
3851 """
3852 from pandas.core.reshape.reshape import pivot
-> 3853 return pivot(self, index=index, columns=columns, values=values)
3854
3855 def stack(self, level=-1, dropna=True):
/Users/robertodias/Anaconda/anaconda2/lib/python2.7/site-packages/pandas/core/reshape/reshape.pyc in pivot(self, index, columns, values)
367 cols = [columns] if index is None else [index, columns]
368 append = index is None
--> 369 indexed = self.set_index(cols, append=append)
370 return indexed.unstack(columns)
371 else:
/Users/robertodias/Anaconda/anaconda2/lib/python2.7/site-packages/pandas/core/frame.pyc in set_index(self, keys, drop, append, inplace, verify_integrity)
2828 names.append(None)
2829 else:
-> 2830 level = frame[col]._values
2831 names.append(col)
2832 if drop:
/Users/robertodias/Anaconda/anaconda2/lib/python2.7/site-packages/pandas/core/frame.pyc in __getitem__(self, key)
1962 return self._getitem_multilevel(key)
1963 else:
-> 1964 return self._getitem_column(key)
1965
1966 def _getitem_column(self, key):
/Users/robertodias/Anaconda/anaconda2/lib/python2.7/site-packages/pandas/core/frame.pyc in _getitem_column(self, key)
1969 # get column
1970 if self.columns.is_unique:
-> 1971 return self._get_item_cache(key)
1972
1973 # duplicate columns & possible reduce dimensionality
/Users/robertodias/Anaconda/anaconda2/lib/python2.7/site-packages/pandas/core/generic.pyc in _get_item_cache(self, item)
1643 res = cache.get(item)
1644 if res is None:
-> 1645 values = self._data.get(item)
1646 res = self._box_item_values(item, values)
1647 cache[item] = res
/Users/robertodias/Anaconda/anaconda2/lib/python2.7/site-packages/pandas/core/internals.pyc in get(self, item, fastpath)
3597 loc = indexer.item()
3598 else:
-> 3599 raise ValueError("cannot label index with a null key")
3600
3601 return self.iget(loc, fastpath=fastpath)
ValueError: cannot label index with a null key