AssertionError: Gaps in blk ref_locs when unstack() dataframe

To get the real data triggered the exception, add extra debug information

Modify ~/anaconda3/lib/python3.6/site-packages/pandas/core/internals.py

add lines to class BlockManager()

def __init__(self)
    print('BlockManager blocks')
    pprint(self.blocks)
    print('BlockManager axes')
    pprint(self.axes)

You will the data:

_unstack_frame level -1 fill_value None 

                 vote  vote2
ballot1 voteId              
NaN     xx      100.0   10.0
False   aaa      50.1    2.0
-1      \n        1.0   18.0
True    NaN      23.0   26.0
b       False    55.0   77.0
a       \        67.0   99.0
c                89.0    9.0
        8        44.0    NaN

Modify ~/anaconda3/lib/python3.6/site-packages/pandas/core/reshape/reshape.py

def __unstack_frame(self, ...)
    from pprint import pprint
    print('_unstack_frame level {} fill_value {} {}'.format(level, fill_value, type(obj)))
    pprint(obj)

You will see data:

BlockManager blocks
(FloatBlock: slice(0, 16, 1), 16 x 8, dtype: float64,)
BlockManager axes
[MultiIndex(levels=[[u'vote', u'vote2'], [False, 8, u'\n', u' ', u'\', u'aaa', u'xx']],
           labels=[[0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1], [-1, 0, 1, 2, 3, 4, 5, 6, -1, 0, 1, 2, 3, 4, 5, 6]],
           names=[None, u'voteId']),
 Index([nan, -1, False, True, u'', u'a', u'b', u'c'], dtype='object', name=u'ballot1')]

I did trigger an exception with another example:

  File "/usr/lib64/python2.7/site-packages/pandas/core/internals.py", line 2902, in _rebuild_blknos_and_blklocs
    raise AssertionError("Gaps in blk ref_locs")
AssertionError: Gaps in blk ref_locs

with debugging info

BlockManager blocks
(FloatBlock: [-1, -1, -1], 3 x 2, dtype: float64,)
BlockManager axes
[Index([aaa, bbb, ccc], dtype='object'), Int64Index([0, 1], dtype='int64')]