This article presents a simple and efficient engine which produces mutations of source code written in C# with helps testing it. The novelty of this engine is that it produces mutations that do not contradict with the specifications of the program. The latter are described by a set of pre- and post-conditions and invariants. The engine comprises two parts, a static analysis and syntactic verification component and a mutation generation component. Preliminary experiments showed that the proposed engine is more efficient than a simple mutations generator in terms of producing only valid mutations according to the specifications posed, thus saving time and effort during testing activities.