title: Getting "ValueError: concat() expects at least one object!" in a simple python code block
body: <p>I ran the following code block in my IPython notebook and got a valueerror. I am not able to figure out whether it is a syntactical error. </p>
  
  <pre><code>import sys
  sys.version
  </code></pre>
  
  <p>would give me</p>
  
  <pre><code>'2.7.9 |Anaconda 2.2.0 (64-bit)| (default, Dec 18 2014, 16:57:52) [MSC v.1500 64 bit (AMD64)]'
  </code></pre>
  
  <p>and on running</p>
  
  <pre><code>from nltk.corpus import brown
  [(genre, word) for genre in brown.categories() for word in brown.words(categories=genre) ]
  </code></pre>
  
  <p>I got the following error:</p>
  
  <pre><code>---------------------------------------------------------------------------
  ValueError                                Traceback (most recent call last)
  &lt;ipython-input-94-884c4187e29a&gt; in &lt;module&gt;()
    1 from nltk.corpus import brown
  ----&gt; 2 [(genre, word) for genre in brown.categories() for word in brown.words(categories=genre) ]
  
  C:\Anaconda\lib\site-packages\nltk\corpus\reader\tagged.pyc in words(self, fileids, categories)
  198     def words(self, fileids=None, categories=None):
  199         return TaggedCorpusReader.words(
  --&gt; 200             self, self._resolve(fileids, categories))
  201     def sents(self, fileids=None, categories=None):
  202         return TaggedCorpusReader.sents(
  
  C:\Anaconda\lib\site-packages\nltk\corpus\reader\tagged.pyc in words(self, fileids)
   81                                         self._para_block_reader,
   82                                         None)
  ---&gt; 83                        for (fileid, enc) in self.abspaths(fileids, True)])
   84 
   85     def sents(self, fileids=None):
  
  C:\Anaconda\lib\site-packages\nltk\corpus\reader\util.pyc in concat(docs)
  412         return docs[0]
  413     if len(docs) == 0:
  --&gt; 414         raise ValueError('concat() expects at least one object!')
  415 
  416     types = set(d.__class__ for d in docs)
  
  ValueError: concat() expects at least one object!"
  </code></pre>
  
  <p>Thanks in advance for all the help.</p>