A small, complete example of the issue

# Your code here
df = pd.DataFrame(columns=['a','b','c'])    
df.groupby('a',as_index=False).agg({'b':'nunique'})

Expected Output

We shouldn't get an Exception

Output of pd.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 2.7.12.final.0 python-bits: 64 OS: Darwin OS-release: 16.0.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: None.None pandas: 0.19.0 nose: 1.3.7 pip: 8.1.2 setuptools: 27.2.0 Cython: 0.25.1 numpy: 1.11.2 scipy: 0.18.1 statsmodels: 0.6.1 xarray: None IPython: 5.1.0 sphinx: 1.4.8 patsy: 0.4.1 dateutil: 2.5.3 pytz: 2016.7 blosc: None bottleneck: 1.1.0 tables: 3.3.0 numexpr: 2.6.1 matplotlib: 1.5.3 openpyxl: 2.4.0 xlrd: 1.0.0 xlwt: 1.1.2 xlsxwriter: 0.9.3 lxml: 3.6.4 bs4: 4.5.1 html5lib: None httplib2: None apiclient: None sqlalchemy: 1.0.9 pymysql: None psycopg2: None jinja2: 2.8 boto: 2.43.0 pandas_datareader: None

i get the following output

Traceback (most recent call last): File "/Users/ilangadassi/Documents/workspace/python/MyTests/test1.py", line 75, in testDfGroup() File "/Users/ilangadassi/Documents/workspace/python/MyTests/test1.py", line 71, in testDfGroup df.groupby('a',as_index=False).agg({'b':'nunique'}) File "/Users/ilangadassi/anaconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 3774, in aggregate return super(DataFrameGroupBy, self).aggregate(arg, args, kwargs) File "/Users/ilangadassi/anaconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 3272, in aggregate result, how = self._aggregate(arg, _level=_level, args, kwargs) File "/Users/ilangadassi/anaconda/lib/python2.7/site-packages/pandas/core/base.py", line 549, in _aggregate result = _agg(arg, _agg_1dim) File "/Users/ilangadassi/anaconda/lib/python2.7/site-packages/pandas/core/base.py", line 500, in _agg result[fname] = func(fname, agg_how) File "/Users/ilangadassi/anaconda/lib/python2.7/site-packages/pandas/core/base.py", line 483, in _agg_1dim return colg.aggregate(how, _level=(_level or 0) + 1) File "/Users/ilangadassi/anaconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 2660, in aggregate return getattr(self, func_or_funcs)(*args, kwargs) File "/Users/ilangadassi/anaconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 3023, in nunique res = out if ids[0] != -1 else out[1:] IndexError: index 0 is out of bounds for axis 0 with size 0

Comment From: jreback

duplicate of #12553

there is a PR #12560 which is stalled if you'd like to pick it up

Comment From: redsum

@jreback , what do you mean by pick it up ? do u want me to suggest a fix ? the fix in PR #12560 , looks good

Comment From: jorisvandenbossche

@redsum With pick it up, we mean to continue the work that is still needed on PR #12560 to get this merged (see the comments over there. Among other things, there are still tests failing)

Comment From: redsum

Hi @jreback i have made the changes. run the tests how do i push the changes?

Comment From: jorisvandenbossche

See http://pandas.pydata.org/pandas-docs/stable/contributing.html#pushing-your-changes on pushing your changes and making a pull request