YouCompleteMe plugin of VIM not working with NLTK

I have recently installed the YouCompleteMe plugin for vim through Vundle. It works really well for Python. Problem starts when I import the third party library nltk. For example if I do the following import:

from nltk.corpus import brown

And then type something like this:

print brown.

I get the following suggestions:

However in idle as well as bpython I get the correct suggestions:

How can I make it work in YouCompleteMe?