Evaluating Recommender System

Hi, I'm trying to evaluate my recommender system in Mahout.
I have boolean data and I'm using *GenericBooleanPrefUserBasedRecommender*
I used precision and recall and it's returning *throwing
java.lang.IllegalArgumentException: Illegal nDCG: NaN**

Does this mean I don't have enough data to evaluate? Is there any way to do
evaluation on this data?

Thank you

Agata Filiana
**http://about.me/dewtraveller* <http://about.me/dewtraveller>*