diff --git a/gtsam/geometry/OrientedPlane3.h b/gtsam/geometry/OrientedPlane3.h
index 5cdaeba93..0f0f20fe5 100644
--- a/gtsam/geometry/OrientedPlane3.h
+++ b/gtsam/geometry/OrientedPlane3.h
@@ -101,7 +101,7 @@ public:
   /// Returns the plane coefficients (a, b, c, d)
   Vector planeCoefficients () const;
 
-  Sphere2 normal () const {
+  inline Sphere2 normal () const {
     return n_;
   }