Memory error during Pandas outer merge

Traceback (most recent call last):
  File "./denormalize.py", line 180, in <module>
    df_join = reduce(lambda left, right: pd.merge(left, right, how='outer', on=['USER_ID']), df_arr)
  File "./denormalize.py", line 180, in <lambda>
    df_join = reduce(lambda left, right: pd.merge(left, right, how='outer', on=['USER_ID']), df_arr)
  File "/usr/local/lib/python2.7/dist-packages/pandas/tools/merge.py", line 38, in merge
    copy=copy, indicator=indicator)
  File "/usr/local/lib/python2.7/dist-packages/pandas/tools/merge.py", line 210, in __init__
    self.join_names) = self._get_merge_keys()
  File "/usr/local/lib/python2.7/dist-packages/pandas/tools/merge.py", line 443, in _get_merge_keys
    left_keys.append(left[lk]._values)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/frame.py", line 1997, in __getitem__
    return self._getitem_column(key)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/frame.py", line 2007, in _getitem_column
    result = self._constructor(self._data.get(key))
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/internals.py", line 3310, in get
    allow_dups=True)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/internals.py", line 3582, in reindex_indexer
    self._consolidate_inplace()
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/internals.py", line 3278, in _consolidate_inplace
    self.blocks = tuple(_consolidate(self.blocks))
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/internals.py", line 4269, in _consolidate
    _can_consolidate=_can_consolidate)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/internals.py", line 4289, in _merge_blocks
    new_values = _vstack([b.values for b in blocks], dtype)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/internals.py", line 4335, in _vstack
    return np.vstack(to_stack)
  File "/usr/local/lib/python2.7/dist-packages/numpy/core/shape_base.py", line 230, in vstack
    return _nx.concatenate([atleast_2d(_m) for _m in tup], 0)
MemoryError