diff --git a/interface/multifluid_mutant.cpp b/interface/multifluid_mutant.cpp
index 74c45b0e9ffdcf7be74c0d0c832ae097ca559754..b878b1434ea225fd594bde2e6661a970ef11ae0c 100644
--- a/interface/multifluid_mutant.cpp
+++ b/interface/multifluid_mutant.cpp
@@ -38,7 +38,7 @@ void add_multifluid_mutant_invariant(py::module& m) {
     m.def("build_multifluid_mutant_invariant", &build_multifluid_mutant_invariant<MultiFluid>);
 
     // Typedef for mutant with the invariant reducing function
-    using Mutant = std::invoke_result_t<decltype(build_multifluid_mutant_invariant<MultiFluid>), std::ref(MultiFluid), std::ref(nlohmann::json)>;
+    using Mutant = std::invoke_result_t<decltype(build_multifluid_mutant_invariant<MultiFluid>), MultiFluid&, nlohmann::json&>;
 
     // Define python wrapper of the mutant class
     auto wMutant = py::class_<Mutant>(m, "MultiFluidMutantInvariant")