[Python] NLTK Lookup Error

While running a Python script using NLTK I got this:

Traceback (most recent call last):
File "cpicklesave.py", line 56, in <module>
pos = nltk.pos_tag(words)
File "/usr/lib/python2.7/site-packages/nltk/tag/__init__.py", line 110, in pos_tag
tagger = PerceptronTagger()
File "/usr/lib/python2.7/site-packages/nltk/tag/perceptron.py", line 140, in __init__
AP_MODEL_LOC = str(find('taggers/averaged_perceptron_tagger/'+PICKLE))
File "/usr/lib/python2.7/site-packages/nltk/data.py", line 641, in find
raise LookupError(resource_not_found)
LookupError:
**********************************************************************
Resource u'taggers/averaged_perceptron_tagger/averaged_perceptro
n_tagger.pickle' not found. Please use the NLTK Downloader to
obtain the resource: >>> nltk.download()
Searched in:
- '/root/nltk_data'
- '/usr/share/nltk_data'
- '/usr/local/share/nltk_data'
- '/usr/lib/nltk_data'
- '/usr/local/lib/nltk_data'
**********************************************************************


Can anyone explain the problem?

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