We propose mutation model checking as a new approach to strengthen formal specifications used for model checking. Inspired by mutation testing, our approach concludes that specifications are not strong enough if they fail to detect faults in purposely mutated models. Our preliminary experiments on two case studies confirm the relevance of the problem: their specification can only detect 40% and 60% of the mutants we randomly generated. As a result, we propose to strengthen the original specification, such that the original model satisfies the strengthened specification but the mutants do not. We sketch a solution framework we plan to implement and discuss research challenges that need to be solved beforehand.