diff --git a/gtsam/nonlinear/NonlinearFactor.h b/gtsam/nonlinear/NonlinearFactor.h
index cd36d4305..a5f06a090 100644
--- a/gtsam/nonlinear/NonlinearFactor.h
+++ b/gtsam/nonlinear/NonlinearFactor.h
@@ -572,6 +572,7 @@ namespace gtsam {
           boost::serialization::base_object<NonlinearFactor>(*this));
       ar & BOOST_SERIALIZATION_NVP(key1_);
       ar & BOOST_SERIALIZATION_NVP(key2_);
+      ar & BOOST_SERIALIZATION_NVP(key3_);
     }
 
   };