I'm learning about the NLTK and my mac
 is working fine except I have trouble with the FreqDist(). (I saw another question about FreqDist() but he was getting a different error message. TypeError: unhashable type: 'list')
Here's an example:

```
>>> from nltk.corpus import brown
>>> news_text = brown.words(categories='news')
>>> fdist = nltk.FreqDist([w.lower() for w in news_text])

Traceback (most recent call last):

`  File "<stdin>", line 1, in <module>`
`NameError: name 'nltk' is not defined`

```

This error message is pretty consistent. I get this message every time I try the FreqDist(). Other commands like - >>> brown.fileids() are fine.

Thanks for your help!