Classical n-grams models lack robustness on unseen events. The literature suggests several smoothing methods: empirically , the most effective of these is the modified Kneser-Ney approach. We propose to improve this back-off model: our method boils down to back-off value reordering, according to the mutual information of the words, and to a new hollow-gram model. Results show that our back-off model yields significant improvements to the baseline, based on the modified Kneser-Ney back-off. We obtain a 0.6% absolute word error rate improvement without acoustic adaptation, and 0.4% after adaptation with a 3xRT ASR system.