diff --git a/Core/include/Acts/Fitter/KalmanFitter.hpp b/Core/include/Acts/Fitter/KalmanFitter.hpp
index 02615f423544948d3f8e4f6c80e0cdf2ef29d1c2..e66cf3ebc5a6e574b08cbb6209cb92c5cdb52a1c 100644
--- a/Core/include/Acts/Fitter/KalmanFitter.hpp
+++ b/Core/include/Acts/Fitter/KalmanFitter.hpp
@@ -399,8 +399,8 @@ class KalmanFitter {
 
     template <typename propagator_state_t, typename stepper_t,
               typename result_t>
-    bool operator()(const result_t& result, propagator_state_t& /*state*/,
-                    const stepper_t& /*stepper*/) const {
+    bool operator()(propagator_state_t& /*state*/, const stepper_t& /*stepper*/,
+                    const result_t& result) const {
       if (!result.result.ok()) {
         return true;
       }