I get a `NotImplementedError` when i run below code. Why?

```
import nltk
from nltk import *
from nltk.tag import stanford
from nltk.tag.stanford import StanfordTagger

st = StanfordTagger('C:\\Python27\\stanford-postagger\\models\\english-bidirectional-distsim.tagger',
                    'C:\\Python27\\stanford-postagger\\stanford-postagger.jar')
print st.tag('What is the airspeed of an unladen swallow ?'.split()) 

```

Error Message:

```
  File "C:\Python27\lib\site-packages\nltk\tag\stanford.py", line 51, in _cmd
    raise NotImplementedError
NotImplementedError

```