diff --git a/sources/reactphysics3d/engine/RK4.cpp b/sources/reactphysics3d/engine/RK4.cpp
index 8b2968ca..cb1602d3 100644
--- a/sources/reactphysics3d/engine/RK4.cpp
+++ b/sources/reactphysics3d/engine/RK4.cpp
@@ -19,6 +19,7 @@
 
 // Libraries
 #include "RK4.h"
+#include <cassert>
 
 // We want to use the ReactPhysics3D namespace
 using namespace reactphysics3d;
@@ -52,7 +53,7 @@ DerivativeBodyState RK4::evaluate(const BodyState& bodyState, const Time& time)
 }
 
 // Compute a derivative body state at time t + dt according to the last derivative body state
-DerivativeBodyState RK4::evaluate(BodyState& bodyState, const Time& time, const Time& timeStep,
+DerivativeBodyState RK4::evaluate(BodyState bodyState, const Time& time, const Time& timeStep,
                                                   const DerivativeBodyState& lastDerivativeBodyState) {
     // Compute the bodyState at time t + dt
     bodyState.computeAtTime(timeStep, lastDerivativeBodyState);
@@ -69,6 +70,7 @@ DerivativeBodyState RK4::evaluate(BodyState& bodyState, const Time& time, const
 
 // Integrate a body state over time. This method use the RK4 integration algorithm
 void RK4::integrate(BodyState& bodyState, const Time& time, const Time& timeStep) {
+
     // Compute the 4 derivatives body states at different time values.
     DerivativeBodyState a = evaluate(bodyState, time);
     DerivativeBodyState b = evaluate(bodyState, time, timeStep*0.5, a);
diff --git a/sources/reactphysics3d/engine/RK4.h b/sources/reactphysics3d/engine/RK4.h
index d3c126d1..090015a5 100644
--- a/sources/reactphysics3d/engine/RK4.h
+++ b/sources/reactphysics3d/engine/RK4.h
@@ -38,7 +38,7 @@ namespace reactphysics3d {
 class RK4 : public IntegrationAlgorithm {
     private :
         DerivativeBodyState evaluate(const BodyState& bodyState, const Time& time);                                 // Compute a derivative body state
-        DerivativeBodyState evaluate(BodyState& bodyState, const Time& time,
+        DerivativeBodyState evaluate(BodyState bodyState, const Time& time,
                                      const Time& timeStep, const DerivativeBodyState& lastDerivativeBodyState);     // Compute a derivative body state
 
     public :