diff --git a/gtsam.h b/gtsam.h
index 60a529541..5a8fcd509 100644
--- a/gtsam.h
+++ b/gtsam.h
@@ -875,6 +875,7 @@ class Cal3_S2 {
   gtsam::Point2 principalPoint() const;
   Vector vector() const;
   Matrix K() const;
+  Matrix matrix() const;
   Matrix matrix_inverse() const;
 
   // enabling serialization functionality