I talked to some friends who worked on that model and they report that this is a problem/mistake in the original paper, i.e. not a mistake you did. There is a unit root together with a fundamental singularity in the model reported in the paper. It has been suggested you should try to contact Andrea Ferrero about the issue if you want to keep working on this model. I have not verified this issue. You also might want to check out the other posts related to this model: