Boolean indexer assginment error
I'm new to Python and Pandas. Can someone help me understand the following. After installing pandas 0.13.0, the following code will crash python. I tested it in pandas 0.12.0, and it's OK.

Thank you!

``` python
import pandas as pd
df = pd.DataFrame(range(100), columns=['a'])
msk = df['a']<5
df['a']=df['a'].map(str)
df['a'][msk]='1'
print msk
```

Here're the current packages versions:

``` python
from pandas.util.print_versions import show_versions
show_versions()


INSTALLED VERSIONS
------------------
Python: 2.7.6.final.0
OS: Linux
Release: 2.6.18-308.4.1.el5
Processor: x86_64
byteorder: little
LC_ALL: C
LANG: en_US.UTF-8

pandas: 0.13.0
Cython: 0.19.2
Numpy: 1.7.1
Scipy: 0.13.2
statsmodels: 0.5.0
    patsy: 0.2.1
scikits.timeseries: Not installed
dateutil: 1.5
pytz: 2013b
bottleneck: Not installed
PyTables: 3.0.0
    numexpr: 2.2.2
matplotlib: 1.3.1
openpyxl: 1.6.2
xlrd: 0.9.2
xlwt: 0.7.5
xlsxwriter: Not installed
sqlalchemy: 0.8.3
lxml: 3.2.3
bs4: 4.3.1
html5lib: Not installed
bigquery: Not installed
apiclient: Not installed

```
