Where operation with setting doesn't work any more
#### Code Sample, a copy-pastable example if possible

```python
df2[df2 > 0] = -df2

```
#### Problem description

In the 10-minute tutorial this code is used, which doesn't work as expected.
The DataFrame contains a Nan value and positive and negative numeric values.

When running the code, an error appears:



---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-52-f7864b9337ed> in <module>()
----> 1 df2[df2 > 0] = -df2

c:\users\asfjhklmd\appdata\local\programs\python\python36-32\lib\site-packages\pandas\core\generic.py in __neg__(self)
   1014             arr = operator.inv(values)
   1015         else:
-> 1016             arr = operator.neg(values)
   1017         return self.__array_wrap__(arr)
   1018 

TypeError: bad operand type for unary -: 'str'

#### Expected Output
A DataFrame containing only negative values and a Nan value

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 32
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.22.0
pytest: None
pip: 9.0.1
setuptools: 28.8.0
Cython: None
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
