diff --git a/SwingTwist.cpp b/SwingTwist.cpp
index 0d10a16..7a7f4a8 100644
--- a/SwingTwist.cpp
+++ b/SwingTwist.cpp
@@ -39,7 +39,7 @@ SwingTwistOf<T> SwingTwistOf<T>::Degrees(float horizontal,
 }
 
 template <typename T>
-Quaternion SwingTwistOf<T>::ToQuaternion() {
+Quaternion SwingTwistOf<T>::ToQuaternion() const {
   Quaternion q = Quaternion::Euler(this->swing.vertical.ToFloat(),
                                    this->swing.horizontal.ToFloat(),
                                    this->twist.ToFloat());
diff --git a/SwingTwist.h b/SwingTwist.h
index f1e72ca..37cfede 100644
--- a/SwingTwist.h
+++ b/SwingTwist.h
@@ -26,7 +26,7 @@ class SwingTwistOf {
   static SwingTwistOf<T> Degrees(float horizontal,
                                  float vertical = 0,
                                  float twist = 0);
-  Quaternion ToQuaternion();
+  Quaternion ToQuaternion() const;
   static SwingTwistOf<T> FromQuaternion(Quaternion q);
 
   const static SwingTwistOf<T> identity;