You may be using a version of Ansible prior to 1.2, the development release.
If you would like to read the 1.1 docs, see here: http://ansible.cc/docs/released/1.1/
Ansible 1.2 is releasing on 6/10 and contains numerous improvements to variable usage, conditionals, and organizational concepts – as well as 30(+?) new modules. You might wish to try it out now, since it’s easy to run from checkout.
(If you are already using 1.2, it sounds like the variable you are using is not defined with the name you think it might be, and I’ll need to see more)
Hope this helps!