Output is NaN

I train my network on a set of data containing car data (year of fabrication, mileage, type, model as input and price as output). I try to predict price for another car but output is NaN. NaN is not even among the values in the training set so this seams like an issue with the brain module.

My code is on GitHib Gist, here: https://gist.github.com/alexnix/146fea914501d283c80635087dd87036