jupyter notebook kernel crashes when doing groupby, having one row with NaN
#### Code Sample, a copy-pastable example if possible

```python

df.groupby('City').count()['Bib'].sort_values(ascending=False).head(20)

```
#### Problem description

Data set has 26630 rows and one row of df["City"] has a NaN. The jupyter notebook kernel crashes.

#### Expected Output

A list of the top 20 cities is expected here.

df[df['City'].notnull()].groupby('City').count()['Bib'].sort_values(ascending=False).head(20)

produces the expected output.

https://github.com/rojour/boston_results

#### Output of ``pd.show_versions()``

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Darwin
OS-release: 16.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.2
nose: 1.3.7
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.11.3
scipy: 0.18.1
statsmodels: 0.6.1
xarray: None
IPython: 5.1.0
sphinx: 1.5.1
patsy: 0.4.1
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 2.0.0
openpyxl: 2.4.1
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.6
lxml: 3.7.2
bs4: 4.5.3
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: 1.1.5
pymysql: None
psycopg2: None
jinja2: 2.9.4
boto: 2.45.0
pandas_datareader: 0.2.1

</details>
