diff --git a/src/math/FGNelderMead.cpp b/src/math/FGNelderMead.cpp
index 637f2f2ea2b124eb4d81675cc56010a1347914bd..f48a209bbef3bf2e516aaeb0f1185fe9b6beac73 100644
--- a/src/math/FGNelderMead.cpp
+++ b/src/math/FGNelderMead.cpp
@@ -30,7 +30,7 @@ namespace JSBSim
 FGNelderMead::FGNelderMead(Function & f, const std::vector<double> & initialGuess,
                            const std::vector<double> & lowerBound,
                            const std::vector<double> & upperBound,
-                           const std::vector<double> initialStepSize, int iterMax,
+                           const std::vector<double> & initialStepSize, int iterMax,
                            double rtol, double abstol, double speed, bool showConvergeStatus,
                            bool showSimplex, bool pause, Callback * callback) :
         m_f(f), m_lowerBound(lowerBound), m_upperBound(upperBound),
diff --git a/src/math/FGNelderMead.h b/src/math/FGNelderMead.h
index 4dee0bfde8f1e82ba216cca2d9fa8960b339f039..ca6188cdd3864738d98b9b62409627a38f7124ca 100644
--- a/src/math/FGNelderMead.h
+++ b/src/math/FGNelderMead.h
@@ -44,7 +44,7 @@ public:
     FGNelderMead(Function & f, const std::vector<double> & initialGuess,
                  const std::vector<double> & lowerBound,
                  const std::vector<double> & upperBound,
-                 const std::vector<double> initialStepSize, int iterMax=2000,
+                 const std::vector<double> & initialStepSize, int iterMax=2000,
                  double rtol=std::numeric_limits<float>::epsilon(),
                  double abstol=std::numeric_limits<float>::epsilon(),
                  double speed = 2.0,