diff --git a/matrix/Matrix.hpp b/matrix/Matrix.hpp
index a910d4fb04..97e1ebf76f 100644
--- a/matrix/Matrix.hpp
+++ b/matrix/Matrix.hpp
@@ -94,7 +94,7 @@ public:
      */
 
 
-    inline Type operator()(size_t i, size_t j) const
+    inline const Type &operator()(size_t i, size_t j) const
     {
         assert(i >= 0);
         assert(i < M);
diff --git a/matrix/Slice.hpp b/matrix/Slice.hpp
index a6e093aca8..faf211ca73 100644
--- a/matrix/Slice.hpp
+++ b/matrix/Slice.hpp
@@ -34,7 +34,7 @@ public:
         assert(y0 + Q <= N);
     }
 
-    Type operator()(size_t i, size_t j) const
+    const Type &operator()(size_t i, size_t j) const
     {
         assert(i >= 0);
         assert(i < P);
diff --git a/matrix/Vector.hpp b/matrix/Vector.hpp
index 40803ba8cc..e9c0d1f1ab 100644
--- a/matrix/Vector.hpp
+++ b/matrix/Vector.hpp
@@ -40,7 +40,7 @@ public:
     {
     }
 
-    inline Type operator()(size_t i) const
+    inline const Type &operator()(size_t i) const
     {
         assert(i >= 0);
         assert(i < M);