On concatenating dataframes: NoneType object is not iterable 
#### Code Sample, a copy-pastable example if possible

```python
import io
import pandas
csv_file = io.StringIO("""datetime,cloudcover_eddh,dewpoint,dewpoint_eddh,humidity,humidity_eddh,lat,lon,precipitation_eddh,pressure_eddh,temperature,temperature_eddh,winddirection_eddh,windgust_eddh,windspeed_eddh
2016-01-01 00:00:00,CAVOC,4.1,3.0,100.0,94.0,53.5443,9.926839999999999,,1023.0,4.1,3.0,160.0,0.0,7.2
2016-01-01 00:00:00,CAVOC,1.7,3.0,96.0,94.0,53.61297,9.98145,,1023.0,2.3,3.0,160.0,0.0,7.2
2016-01-01 00:00:00,CAVOC,2.4,3.0,98.0,94.0,53.57735,10.09428,,1023.0,2.7,3.0,160.0,0.0,7.2
2016-01-01 00:00:00,CAVOC,3.0,3.0,94.0,94.0,53.68849,10.1335,,1023.0,3.9,3.0,160.0,0.0,7.2
2016-01-01 00:00:00,CAVOC,4.2,3.0,76.0,94.0,53.6608,10.06555,,1023.0,,3.0,160.0,0.0,7.2
2016-01-01 00:00:00,CAVOC,3.0,3.0,100.0,94.0,53.43252,10.297989999999999,,1023.0,3.0,3.0,160.0,0.0,7.2
2016-01-01 00:00:00,CAVOC,1.9,3.0,92.0,94.0,53.68937,10.13025,,1023.0,3.1,3.0,160.0,0.0,7.2
2016-01-01 00:00:00,CAVOC,3.5,3.0,100.0,94.0,53.6344,9.966560000000001,,1023.0,3.5,3.0,160.0,0.0,7.2
2016-01-01 00:00:00,CAVOC,3.6,3.0,99.0,94.0,53.46402,9.89157,,1023.0,3.7,3.0,160.0,0.0,7.2""")
data_df = pandas.read_csv(csv_file, parse_dates=["datetime"], index_col="datetime")
cloud_cover_df = pandas.get_dummies(data_df.cloudcover_eddh, prefix="cloudcover_eddh")
df_hour = pandas.get_dummies(data_df.index.hour, prefix="hour")
data_df.reset_index(inplace=True, drop=True)
data_df = pandas.concat([
   data_df,
   df_hour,
    cloud_cover_df
], axis=1, ignore_index=True)

```
#### Problem description

I expect that the three dataframes are concatenated as wished. I know that the indices correspond so I do not need pandas to check that for me. Instead I get the following error message:

```Traceback (most recent call last):
  File "<stdin>", line 5, in <module>
  File "/informatik2/students/home/1kastner/.local/lib/python3.5/site-packages/pandas/core/reshape/concat.py", line 207, in concat
    return op.get_result()
  File "/informatik2/students/home/1kastner/.local/lib/python3.5/site-packages/pandas/core/reshape/concat.py", line 401, in get_result
    indexers[ax] = obj_labels.reindex(new_labels)[1]
  File "/informatik2/students/home/1kastner/.local/lib/python3.5/site-packages/pandas/core/indexes/base.py", line 2887, in reindex
    indexer, missing = self.get_indexer_non_unique(target)
  File "/informatik2/students/home/1kastner/.local/lib/python3.5/site-packages/pandas/core/indexes/base.py", line 2708, in get_indexer_non_unique
    indexer, missing = self._engine.get_indexer_non_unique(tgt_values)
  File "pandas/_libs/index.pyx", line 325, in pandas._libs.index.IndexEngine.get_indexer_non_unique (pandas/_libs/index.c:7722)
TypeError: 'NoneType' object is not iterable
```

#### Output of ``pd.show_versions()``
<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.10.0-33-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.3
pytest: None
pip: 9.0.1
setuptools: 36.0.1
Cython: None
numpy: 1.13.1
scipy: 0.17.0
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 1.5.1
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: 0.7.3
lxml: None
bs4: 4.4.1
html5lib: 0.999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
pandas_gbq: None
pandas_datareader: None

</details>
