diff --git a/sophus/sim2.hpp b/sophus/sim2.hpp
index c69cb8703..425765dd2 100644
--- a/sophus/sim2.hpp
+++ b/sophus/sim2.hpp
@@ -357,7 +357,7 @@ class Sim2Base {
 
   /// Setter of complex number using rotation matrix ``R``, leaves scale as is.
   ///
-  SOPHUS_FUNC void setRotationMatrix(Matrix2<Scalar>& R) {
+  SOPHUS_FUNC void setRotationMatrix(Matrix2<Scalar> const& R) {
     rxso2().setRotationMatrix(R);
   }
 
diff --git a/sophus/sim3.hpp b/sophus/sim3.hpp
index a7a6eb3a1..e1f6e72ef 100644
--- a/sophus/sim3.hpp
+++ b/sophus/sim3.hpp
@@ -355,7 +355,7 @@ class Sim3Base {
 
   /// Setter of quaternion using rotation matrix ``R``, leaves scale as is.
   ///
-  SOPHUS_FUNC void setRotationMatrix(Matrix3<Scalar>& R) {
+  SOPHUS_FUNC void setRotationMatrix(Matrix3<Scalar> const& R) {
     rxso3().setRotationMatrix(R);
   }