diff --git a/include/teqp/derivs.hpp b/include/teqp/derivs.hpp
index 9fa6602d54f97c957ba76d67cb2eb572948d5268..da72c1ff4c9dc97d3038cf2706968451aa0c8549 100644
--- a/include/teqp/derivs.hpp
+++ b/include/teqp/derivs.hpp
@@ -981,7 +981,7 @@ struct IsochoricDerivatives{
             return forceeval(model.alphar(T, rhotot, molefrac) * model.R(molefrac) * T * rhotot);
         };
         auto der = derivatives(wrapper, along(1), at(sigma));
-        VectorType ret(rhovec.size());
+        VectorType ret(der.size());
         for (auto i = 0; i < ret.size(); ++i){ ret[i] = der[i];}
         return ret;
     }