diff --git a/include/teqp/derivs.hpp b/include/teqp/derivs.hpp
index e1169bfcb0f0a3bf556ca1346b728f4234bf0827..6a38256f8d14fe848f944743567c0350197a38ac 100644
--- a/include/teqp/derivs.hpp
+++ b/include/teqp/derivs.hpp
@@ -85,7 +85,7 @@ struct TDXDerivatives {
     }
 
     template<ADBackends be = ADBackends::autodiff>
-    static auto get_Ar01(const Model& model, const Scalar&T, const Scalar &rho, const VectorType& molefrac) {
+    static Scalar get_Ar01(const Model& model, const Scalar&T, const Scalar &rho, const VectorType& molefrac){
         if constexpr(be == ADBackends::complex_step){
             double h = 1e-100;
             auto der = model.alphar(T, std::complex<Scalar>(rho, h), molefrac).imag() / h;