diff --git a/include/Geometry/Spline.inl b/include/Geometry/Spline.inl
index 0f25291..00383e8 100644
--- a/include/Geometry/Spline.inl
+++ b/include/Geometry/Spline.inl
@@ -181,7 +181,7 @@ namespace detail
 template<std::size_t I, std::size_t J, typename... Types>
 struct evalSplineHelper
 {
-    std::tuple<Types...> operator()(
+    std::tuple<Types...>& operator()(
         std::vector<std::tuple<Types...>> const& controlPoints,
         std::vector<float> const& nodes,
         int degree,
@@ -204,7 +204,7 @@ struct evalSplineHelper
 template<std::size_t J, typename... Types>
 struct evalSplineHelper<0,J,Types...>
 {
-    std::tuple<Types...> operator()(
+    std::tuple<Types...>& operator()(
         std::vector<std::tuple<Types...>> const& controlPoints,
         std::vector<float> const& nodes,
         int degree,
@@ -232,7 +232,7 @@ template<std::size_t I, std::size_t J, typename... Types>
 struct evalDerivativeSplineHelper
 {
 
-    std::tuple<Types...> operator()(std::vector<std::tuple<Types...>> const& controlPoints , std::vector<float> const& nodes, int degree, float f, std::tuple<Types...>& tuple)
+    std::tuple<Types...>& operator()(std::vector<std::tuple<Types...>> const& controlPoints , std::vector<float> const& nodes, int degree, float f, std::tuple<Types...>& tuple)
     {
         std::get<J>(tuple) = 0;
 
@@ -258,7 +258,7 @@ struct evalDerivativeSplineHelper
 template<std::size_t J,typename... Types>
 struct evalDerivativeSplineHelper<0,J,Types...>
 {
-    std::tuple<Types...> operator()(std::vector<std::tuple<Types...>> const& controlPoints , std::vector<float> const& nodes, int degree, float f, std::tuple<Types...>& tuple)
+    std::tuple<Types...>& operator()(std::vector<std::tuple<Types...>> const& controlPoints , std::vector<float> const& nodes, int degree, float f, std::tuple<Types...>& tuple)
     {
         // Nothing to do here
         return tuple;